patch 8.1.0949: MS-windows defines GUI macros different than other systems

Problem:    MS-windows defines GUI macros different than other systems.
Solution:   Swap FEAT_GUI and FEAT_GUI_MSWIN. (Hirohito Higashi, closes #3996)
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 391277f..29d8b0e 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -791,8 +791,8 @@
 
 !if "$(GUI)" == "yes"
 SUBSYSTEM = windows
-CFLAGS = $(CFLAGS) -DFEAT_GUI
-RCFLAGS = $(RCFLAGS) -DFEAT_GUI
+CFLAGS = $(CFLAGS) -DFEAT_GUI_MSWIN
+RCFLAGS = $(RCFLAGS) -DFEAT_GUI_MSWIN
 VIM = g$(VIM)
 GUI_INCL = \
 	gui.h