Ensure Makefile.config gets Windows-style prefix

Prefix may be passed to configure using a Cygwin-style PATH (e.g.
`./configure --prefix ~/local`). Use cygpath to ensure that the correct
version gets written to Makefile.config, or the resulting compiler has
an invalid default stdlib path.

Changes

@@ -71,6 +71,10 @@ Working version
 
 ### Build system:
 
+- #9136: Don't propagate Cygwin-style prefix from configure to Makefile.config
+  on Windows ports.
+  (David Allsopp, review by Sébastien Hinderer)
+
 ### Bug fixes:
 
 - #7683, #1499: Fixes one case where the evaluation order in native-code

README.win32.adoc

@@ -202,9 +202,7 @@ for 32-bit, or:
 
         ./configure --build=x86_64-unknown-cygwin --host=x86_64-pc-windows
 
-for 64-bit. Then, edit `Makefile.config` as needed, following the comments in
-this file. Normally, the only variable that needs to be changed is `PREFIX`,
-which indicates where to install everything.
+for 64-bit.
 
 Finally, use `make` to build the system, e.g.
 
@@ -269,9 +267,7 @@ for 32-bit, or:
 
         ./configure --build=x86_64-unknown-cygwin --host=x86_64-w64-mingw32
 
-for 64-bit. Then, edit `Makefile.config` as needed, following the comments in
-this file. Normally, the only variable that needs to be changed is `PREFIX`,
-which indicates where to install everything.
+for 64-bit.
 
 Finally, use `make` to build the system, e.g.
 

configure

@@ -16832,6 +16832,16 @@ if test x"$prefix" = "xNONE"; then :
   *) :
      ;;
 esac
+else
+  if test x"$unix_or_win32" = "xwin32" \
+          && test "$host_vendor-$host_os" != "$build_vendor-$build_os" ; then :
+  case $build in #(
+  *-pc-cygwin) :
+    prefix=`cygpath -m "$prefix"` ;; #(
+  *) :
+     ;;
+esac
+fi
 fi
 
 # Define a few macros that were defined in config/m-nt.h

configure.ac

@@ -1781,7 +1781,11 @@ AS_IF([test x"$prefix" = "xNONE"],
     [i686-w64-mingw32], [prefix='C:/ocamlmgw'],
     [x86_64-w64-mingw32], [prefix='C:/ocamlmgw64'],
     [i686-pc-windows], [prefix='C:/ocamlms'],
-    [x86_64-pc-windows], [prefix='C:/ocamlms64'])])
+    [x86_64-pc-windows], [prefix='C:/ocamlms64'])],
+  [AS_IF([test x"$unix_or_win32" = "xwin32" \
+          && test "$host_vendor-$host_os" != "$build_vendor-$build_os" ],
+    [AS_CASE([$build],
+      [*-pc-cygwin], [prefix=`cygpath -m "$prefix"`])])])
 
 # Define a few macros that were defined in config/m-nt.h
 # but whose value is not guessed properly by configure