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