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/Make_mvc.mak b/src/Make_mvc.mak
index c81fbd2..391277f 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -791,8 +791,8 @@
 
 !if "$(GUI)" == "yes"
 SUBSYSTEM = windows
-CFLAGS = $(CFLAGS) -DFEAT_GUI_W32
-RCFLAGS = $(RCFLAGS) -DFEAT_GUI_W32
+CFLAGS = $(CFLAGS) -DFEAT_GUI
+RCFLAGS = $(RCFLAGS) -DFEAT_GUI
 VIM = g$(VIM)
 GUI_INCL = \
 	gui.h