Escape slashes in paths passed to mingw-gcc

Avoids Msys path mangling that turns *nix paths (such as /usr/include) into
DOS-style absolute paths (such as c:/mingw/msys/1.0/include).
Allows mingw-built pkg-config to pass check-cflags.

Makefile.am

@@ -21,12 +21,21 @@ EXTRA_DIST = $(m4_DATA) $(man_MANS) README.win32
 bin_PROGRAMS = pkg-config
 AM_CFLAGS=@WARN_CFLAGS@
 
+if NATIVE_WIN32
+INCLUDES= \
+	-DPKG_CONFIG_PC_PATH="\"$(subst /,\/,$(pc_path))\"" \
+	-DPKG_CONFIG_SYSTEM_INCLUDE_PATH="\"$(subst /,\/,$(system_include_path))\"" \
+	-DPKG_CONFIG_SYSTEM_LIBRARY_PATH="\"$(subst /,\/,$(system_library_path))\"" \
+	@GLIB_CFLAGS@ \
+	$(popt_includes)
+else
 INCLUDES= \
 	-DPKG_CONFIG_PC_PATH="\"$(pc_path)\"" \
 	-DPKG_CONFIG_SYSTEM_INCLUDE_PATH="\"$(system_include_path)\"" \
 	-DPKG_CONFIG_SYSTEM_LIBRARY_PATH="\"$(system_library_path)\"" \
 	@GLIB_CFLAGS@ \
 	$(popt_includes)
+endif
 
 pkg_config_SOURCES= \
 	pkg.h \

configure.ac

@@ -107,7 +107,7 @@ AC_DEFINE_UNQUOTED(ENABLE_INDIRECT_DEPS, `test $use_indirect_deps = no; echo $?`
 AC_SUBST(use_indirect_deps)
 
 AC_MSG_CHECKING([for Win32])
-case "$host" in
+case "$build" in
   *-*-mingw*)
     native_win32=yes
   ;;
@@ -116,6 +116,7 @@ case "$host" in
   ;;
 esac
 AC_MSG_RESULT([$native_win32])
+AM_CONDITIONAL(NATIVE_WIN32, [test "x$native_win32" = xyes])
 
 if test "x$GLIB_CFLAGS" = "x" && test "x$GLIB_LIBS" = "x"; then
    AC_CHECK_PROGS([PKG_CONFIG], [pkg-config], [])