Build system: fix `pwd` issues on Windows

Some parts of the build system require that paths are what msys2 calls
"mixed style":
  * forwards slashes
  * absolute paths starting with a drive letter followed by a colon
    (e.g. "C:")

The removal of ghc-pwd in 4c56ad36 changed $(TOP) from mixed style to
unix style, resulting in a broken Windows build for some.

Differential Revision: https://phabricator.haskell.org/D1752

aclocal.m4

@@ -1739,7 +1739,15 @@ AC_DEFUN([FP_CURSES],
 
 AC_DEFUN([FP_FIND_ROOT],[
 AC_MSG_CHECKING(for path to top of build tree)
-    hardtop=`pwd`
+    if test "$windows" = YES
+    then
+      dnl Make sure this is a c:/foo/bar (mixed) style path. Some parts of
+      dnl the build system might depend on it (such as the sed expression
+      dnl `"s|$(TOP)/||i"` in addCFileDeps in rules/build-dependencies.mk).
+      hardtop=$(cygpath -m "$(pwd)")
+    else
+      hardtop=$(pwd)
+    fi
 
     dnl Remove common automounter nonsense
     hardtop=`echo $hardtop | sed 's|^/tmp_mnt.*\(/local/.*\)$|\1|' | sed 's|^/tmp_mnt/|/|'`

configure.ac

@@ -26,8 +26,6 @@ AC_INIT([The Glorious Glasgow Haskell Compilation System], [8.1], [glasgow-haske
 # checkout), then we ship a file 'VERSION' containing the full version
 # when the source distribution was created.
 
-FP_FIND_ROOT
-
 if test ! -f mk/config.h.in; then
    echo "mk/config.h.in doesn't exist: perhaps you haven't run 'perl boot'?"
    exit 1
@@ -260,6 +258,9 @@ AC_ARG_WITH(hc,
 )
 AC_SUBST(WithHc)
 
+# Requires FPTOOLS_SET_PLATFORM_VARS to be run first.
+FP_FIND_ROOT
+
 fail() {
     echo >&2
     echo "$1" >&2

distrib/configure.ac.in

@@ -6,8 +6,6 @@ dnl
 
 AC_INIT([The Glorious Glasgow Haskell Compilation System], [@ProjectVersion@], [glasgow-haskell-bugs@haskell.org], [ghc])
 
-FP_FIND_ROOT
-
 dnl--------------------------------------------------------------------
 dnl * Deal with arguments telling us gmp is somewhere odd
 dnl--------------------------------------------------------------------
@@ -17,6 +15,9 @@ FP_GMP
 bootstrap_target=@TargetPlatform@
 FPTOOLS_SET_PLATFORM_VARS
 
+# Requires FPTOOLS_SET_PLATFORM_VARS to be run first.
+FP_FIND_ROOT
+
 # ToDo: if Stage1Only=YES, should be YES
 CrossCompiling=NO
 CrossCompilePrefix=""