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/os_w32exe.c b/src/os_w32exe.c
index f96bff4..197d65d 100644
--- a/src/os_w32exe.c
+++ b/src/os_w32exe.c
@@ -22,7 +22,7 @@
 
 /* cproto doesn't create a prototype for main() */
 int _cdecl
-#if defined(FEAT_GUI_W32)
+#if defined(FEAT_GUI_MSWIN)
 VimMain
 #else
     main
@@ -104,7 +104,7 @@
     pSaveInst = SaveInst;
 # endif
     pmain =
-# if defined(FEAT_GUI_W32)
+# if defined(FEAT_GUI_MSWIN)
     //&& defined(__MINGW32__)
 	VimMain
 # else