Fix python_includespec on Windows at configure time

By converting to using forward slashes at configure time we avoid
having to repeat the logic anywhere that this is needed, such as
in transforms modules for plpython.

config/python.m4

@@ -44,6 +44,9 @@ if a == b:
     print(a)
 else:
     print(a + ' ' + b)"`
+if test "$PORTNAME" = win32 ; then
+    python_includespec=`echo $python_includespec | sed 's,[\],/,g'`
+fi
 AC_MSG_RESULT([$python_includespec])
 
 AC_SUBST(python_majorversion)[]dnl

configure

@@ -7487,6 +7487,9 @@ if a == b:
     print(a)
 else:
     print(a + ' ' + b)"`
+if test "$PORTNAME" = win32 ; then
+    python_includespec=`echo $python_includespec | sed 's,\,/,g'`
+fi
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $python_includespec" >&5
 $as_echo "$python_includespec" >&6; }
 

src/pl/plpython/Makefile

@@ -5,14 +5,12 @@ top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
 
-# Windows needs to convert backslashed paths to normal slashes,
-# and we have to remove -lpython from the link since we are building our own
+# On Windows we have to remove -lpython from the link since we are
+# building our own
 ifeq ($(PORTNAME), win32)
-python_includespec := $(subst \,/,$(python_includespec))
 override python_libspec =
 endif
 
-
 override CPPFLAGS := -I. -I$(srcdir) $(python_includespec) $(CPPFLAGS)
 
 rpathdir = $(python_libdir)