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/proto.h b/src/proto.h
index c19d155..43192ff 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -38,7 +38,7 @@
 # if defined(UNIX) || defined(VMS)
 #  include "os_unix.pro"
 # endif
-# ifdef WIN3264
+# ifdef MSWIN
 #  include "os_win32.pro"
 #  include "os_mswin.pro"
 #  include "winclip.pro"
@@ -266,7 +266,7 @@
 extern char_u *vimpty_getenv(const char_u *string);	/* in misc2.c */
 #   endif
 #  endif
-#  ifdef FEAT_GUI_W32
+#  ifdef FEAT_GUI_MSWIN
 #   include "gui_w32.pro"
 #  endif
 #  ifdef FEAT_GUI_GTK