diff --git a/src/Pure/Admin/build_polyml.scala b/src/Pure/Admin/build_polyml.scala
--- a/src/Pure/Admin/build_polyml.scala
+++ b/src/Pure/Admin/build_polyml.scala
@@ -1,314 +1,312 @@
 /*  Title:      Pure/Admin/build_polyml.scala
     Author:     Makarius
 
 Build Poly/ML from sources.
 */
 
 package isabelle
 
 
 import scala.util.matching.Regex
 
 
 object Build_PolyML
 {
   /** platform-specific build **/
 
   sealed case class Platform_Info(
     options: List[String] = Nil,
     setup: String = "",
     copy_files: List[String] = Nil,
     ldd_pattern: Option[(String, Regex)] = None)
 
   private val platform_info = Map(
     "linux" ->
       Platform_Info(
         options = List("LDFLAGS=-Wl,-rpath,_DUMMY_"),
         ldd_pattern = Some(("ldd", """\s*libgmp.*=>\s*(\S+).*""".r))),
     "darwin" ->
       Platform_Info(
         options =
           List("--build=x86_64-darwin", "CFLAGS=-arch x86_64 -O3 -I../libffi/include",
             "CXXFLAGS=-arch x86_64 -O3 -I../libffi/include", "CCASFLAGS=-arch x86_64",
             "LDFLAGS=-segprot POLY rwx rwx"),
         setup = "PATH=/usr/bin:/bin:/usr/sbin:/sbin",
         ldd_pattern = Some(("otool -L", """\s*(\S+lib(?:polyml|gmp).*dylib).*""".r))),
     "windows" ->
       Platform_Info(
         options =
           List("--host=x86_64-w64-mingw32", "CPPFLAGS=-I/mingw64/include", "--disable-windows-gui"),
-        setup =
-          """PATH=/usr/bin:/bin:/mingw64/bin
-            export CONFIG_SITE=/mingw64/etc/config.site""",
+        setup = MinGW.environment_export,
         copy_files =
           List("$MSYS/mingw64/bin/libgcc_s_seh-1.dll",
             "$MSYS/mingw64/bin/libgmp-10.dll",
             "$MSYS/mingw64/bin/libstdc++-6.dll",
             "$MSYS/mingw64/bin/libwinpthread-1.dll")))
 
   def build_polyml(
     root: Path,
     sha1_root: Option[Path] = None,
     progress: Progress = new Progress,
     arch_64: Boolean = false,
     options: List[String] = Nil,
     msys_root: Option[Path] = None)
   {
     if (!((root + Path.explode("configure")).is_file && (root + Path.explode("PolyML")).is_dir))
       error("Bad Poly/ML root directory: " + root)
 
     val platform = Isabelle_Platform.self
     val platform_arch =
       if (arch_64) platform.arch_64
       else if (platform.is_intel) "x86_64_32"
       else platform.arch_32
 
     val polyml_platform = platform_arch + "-" + platform.os_name
     val sha1_platform = platform.arch_64 + "-" + platform.os_name
 
     val info =
       platform_info.getOrElse(platform.os_name,
         error("Bad OS platform: " + quote(platform.os_name)))
 
     val settings =
       msys_root match {
         case None if platform.is_windows =>
           error("Windows requires specification of msys root directory")
         case None => Isabelle_System.settings()
         case Some(msys) => Isabelle_System.settings() + ("MSYS" -> msys.expand.implode)
       }
 
     if (platform.is_linux && !Isabelle_System.bash("chrpath -v").ok) {
       error("""Missing "chrpath" executable on Linux""")
     }
 
 
     /* bash */
 
     def bash(
       cwd: Path, script: String, redirect: Boolean = false, echo: Boolean = false): Process_Result =
     {
       val script1 =
         msys_root match {
           case None => script
           case Some(msys) =>
             File.bash_path(msys + Path.explode("usr/bin/bash")) + " -c " + Bash.string(script)
         }
       progress.bash(script1, cwd = cwd.file, redirect = redirect, echo = echo)
     }
 
 
     /* configure and make */
 
     val configure_options =
       List("--disable-shared", "--enable-intinf-as-int", "--with-gmp") :::
         info.options ::: options ::: (if (arch_64) Nil else List("--enable-compact32bit"))
 
     bash(root,
       info.setup + "\n" +
       """
         [ -f Makefile ] && make distclean
         {
           ./configure --prefix="$PWD/target" """ + Bash.strings(configure_options) + """
           rm -rf target
           make compiler && make compiler && make install
         } || { echo "Build failed" >&2; exit 2; }
       """, redirect = true, echo = true).check
 
     val ldd_files =
     {
       info.ldd_pattern match {
         case Some((ldd, pattern)) =>
           val lines = bash(root, ldd + " target/bin/poly").check.out_lines
           for { line <- lines; List(lib) <- pattern.unapplySeq(line) } yield lib
         case None => Nil
       }
     }
 
 
     /* sha1 library */
 
     val sha1_files =
       if (sha1_root.isDefined) {
         val dir1 = sha1_root.get
         bash(dir1, "./build " + sha1_platform, redirect = true, echo = true).check
 
         val dir2 = dir1 + Path.explode(sha1_platform)
         File.read_dir(dir2).map(entry => dir2.implode + "/" + entry)
       }
       else Nil
 
 
     /* target */
 
     val target = Path.explode(polyml_platform)
     Isabelle_System.rm_tree(target)
     Isabelle_System.make_directory(target)
 
     for (file <- info.copy_files ::: ldd_files ::: sha1_files)
       File.copy(Path.explode(file).expand_env(settings), target)
 
     for {
       d <- List("target/bin", "target/lib")
       dir = root + Path.explode(d)
       entry <- File.read_dir(dir)
     } File.move(dir + Path.explode(entry), target)
 
 
     /* poly: library path */
 
     if (platform.is_linux) {
       bash(target, "chrpath -r '$ORIGIN' poly", echo = true).check
     }
     else if (platform.is_macos) {
       for (file <- ldd_files) {
         bash(target,
           """install_name_tool -change """ + Bash.string(file) + " " +
             Bash.string("@executable_path/" + Path.explode(file).file_name) + " poly").check
       }
     }
 
 
     /* polyc: directory prefix */
 
     val Header = "#! */bin/sh".r
     File.change(target + Path.explode("polyc"), txt =>
       split_lines(txt) match {
         case Header() :: lines =>
           val lines1 =
             lines.map(line =>
               if (line.startsWith("prefix=")) "prefix=\"$(cd \"$(dirname \"$0\")\"; pwd)\""
               else if (line.startsWith("BINDIR=")) "BINDIR=\"$prefix\""
               else if (line.startsWith("LIBDIR=")) "LIBDIR=\"$prefix\""
               else line)
           cat_lines("#!/usr/bin/env bash" ::lines1)
         case lines =>
           error(cat_lines("Cannot patch polyc -- undetected header:" :: lines.take(3)))
       }
     )
   }
 
 
 
   /** skeleton for component **/
 
   private def extract_sources(source_archive: Path, component_dir: Path) =
   {
     if (source_archive.get_ext == "zip") {
       Isabelle_System.bash(
         "unzip -x " + File.bash_path(source_archive.absolute), cwd = component_dir.file).check
     }
     else {
       Isabelle_System.gnutar("-xzf " + File.bash_path(source_archive), dir = component_dir).check
     }
 
     val src_dir = component_dir + Path.explode("src")
     File.read_dir(component_dir) match {
       case List(s) => File.move(component_dir + Path.basic(s), src_dir)
       case _ => error("Source archive contains multiple directories")
     }
 
     val lines = split_lines(File.read(src_dir + Path.explode("RootX86.ML")))
     val ml_files =
       for {
         line <- lines
         rest <- Library.try_unprefix("use", line)
       } yield "ML_file" + rest
 
     File.write(src_dir + Path.explode("ROOT.ML"),
 """(* Poly/ML Compiler root file.
 
 When this file is open in the Prover IDE, the ML files of the Poly/ML
 compiler can be explored interactively. This is a separate copy: it does
 not affect the running ML session. *)
 """ + ml_files.mkString("\n", "\n", "\n"))
   }
 
   def build_polyml_component(
     source_archive: Path,
     component_dir: Path,
     sha1_root: Option[Path] = None)
   {
     Isabelle_System.new_directory(component_dir)
     extract_sources(source_archive, component_dir)
 
     File.copy(Path.explode("~~/Admin/polyml/README"), component_dir)
 
     val etc_dir = Isabelle_System.make_directory(component_dir + Path.explode("etc"))
     File.copy(Path.explode("~~/Admin/polyml/settings"), etc_dir)
 
     sha1_root match {
       case Some(dir) =>
         Mercurial.repository(dir).archive(File.standard_path(component_dir + Path.explode("sha1")))
       case None =>
     }
   }
 
 
 
   /** Isabelle tool wrappers **/
 
   val isabelle_tool1 =
     Isabelle_Tool("build_polyml", "build Poly/ML from sources", args =>
     {
       var msys_root: Option[Path] = None
       var arch_64 = Isabelle_Platform.self.is_arm
       var sha1_root: Option[Path] = None
 
       val getopts = Getopts("""
 Usage: isabelle build_polyml [OPTIONS] ROOT [CONFIGURE_OPTIONS]
 
   Options are:
     -M DIR       msys root directory (for Windows)
     -m ARCH      processor architecture (32 or 64, default: """ +
         (if (arch_64) "64" else "32") + """)
     -s DIR       sha1 sources, see https://isabelle.sketis.net/repos/sha1
 
   Build Poly/ML in the ROOT directory of its sources, with additional
   CONFIGURE_OPTIONS (e.g. --without-gmp).
 """,
         "M:" -> (arg => msys_root = Some(Path.explode(arg))),
         "m:" ->
           {
             case "32" => arch_64 = false
             case "64" => arch_64 = true
             case bad => error("Bad processor architecture: " + quote(bad))
           },
         "s:" -> (arg => sha1_root = Some(Path.explode(arg))))
 
       val more_args = getopts(args)
       val (root, options) =
         more_args match {
           case root :: options => (Path.explode(root), options)
           case Nil => getopts.usage()
         }
       build_polyml(root, sha1_root = sha1_root, progress = new Console_Progress,
         arch_64 = arch_64, options = options, msys_root = msys_root)
     })
 
   val isabelle_tool2 =
     Isabelle_Tool("build_polyml_component", "make skeleton for Poly/ML component", args =>
     {
       var sha1_root: Option[Path] = None
 
       val getopts = Getopts("""
 Usage: isabelle build_polyml_component [OPTIONS] SOURCE_ARCHIVE COMPONENT_DIR
 
   Options are:
     -s DIR       sha1 sources, see https://isabelle.sketis.net/repos/sha1
 
   Make skeleton for Poly/ML component, based on official source archive
   (zip or tar.gz).
 """,
         "s:" -> (arg => sha1_root = Some(Path.explode(arg))))
 
       val more_args = getopts(args)
 
       val (source_archive, component_dir) =
         more_args match {
           case List(archive, dir) => (Path.explode(archive), Path.explode(dir))
           case _ => getopts.usage()
         }
       build_polyml_component(source_archive, component_dir, sha1_root = sha1_root)
     })
 }
diff --git a/src/Pure/System/mingw.scala b/src/Pure/System/mingw.scala
--- a/src/Pure/System/mingw.scala
+++ b/src/Pure/System/mingw.scala
@@ -1,52 +1,54 @@
 /*  Title:      Pure/System/mingw.scala
     Author:     Makarius
 
 Support for MSYS2/MinGW64 on Windows.
 */
 
 package isabelle
 
 
 object MinGW
 {
-  def environment: List[(String, String)] =
-    List("PATH" -> "/usr/bin:/bin:/mingw64/bin", "CONFIG_SITE" -> "/mingw64/etc/config.site")
+  def environment: List[String] =
+    List("PATH=/usr/bin:/bin:/mingw64/bin", "CONFIG_SITE=/mingw64/etc/config.site")
 
   def environment_prefix: String =
-    (for ((a, b) <- environment) yield Bash.string(a) + "=" + Bash.string(b))
-      .mkString("/usr/bin/env ", " ", " ")
+    environment.map(Bash.string).mkString("/usr/bin/env ", " ", " ")
+
+  def environment_export: String =
+    environment.map(a => "export " + Bash.string(a)).mkString("", "\n", "\n")
 
   val none: MinGW = new MinGW(None)
   def root(path: Path) = new MinGW(Some(path))
 }
 
 class MinGW private(val root: Option[Path])
 {
   override def toString: String =
     root match {
       case None => "MinGW.none"
       case Some(msys_root) => "MinGW.root(" + msys_root.toString + ")"
     }
 
   def bash_command(command: String): String =
     root match {
       case None => command
       case Some(msys_root) =>
         File.bash_path(msys_root + Path.explode("usr/bin/bash")) +
           " -c " + Bash.string(MinGW.environment_prefix + command)
     }
 
   def get_root: Path =
     if (!Platform.is_windows) error("Windows platform required")
     else if (root.isEmpty) error("Windows platform requires msys/mingw root specification")
     else root.get
 
   def check
   {
     if (Platform.is_windows) {
       get_root
       try { require(Isabelle_System.bash(bash_command("uname -s")).check.out.startsWith("MSYS")) }
       catch { case ERROR(_) => error("Bad msys/mingw installation " + get_root) }
     }
   }
 }