patch 8.1.0941: macros for MS-Windows are inconsistent

Problem:    Macros for MS-Windows are inconsistent, using "32", "3264 and
            others.
Solution:   Use MSWIN for all MS-Windows builds.  Use FEAT_GUI_MSWIN for the
            GUI build. (Hirohito Higashi, closes #3932)
diff --git a/src/search.c b/src/search.c
index 9a47ed4..20f6299 100644
--- a/src/search.c
+++ b/src/search.c
@@ -1246,7 +1246,7 @@
     }
     if (options & SEARCH_REV)
     {
-#ifdef WIN32
+#ifdef MSWIN
 	/* There is a bug in the Visual C++ 2.2 compiler which means that
 	 * dirc always ends up being '/' */
 	dirc = (dirc == '/')  ?  '?'  :  '/';