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_bc5.mak b/src/Make_bc5.mak
index 30fe727..04f7885 100644
--- a/src/Make_bc5.mak
+++ b/src/Make_bc5.mak
@@ -405,7 +405,7 @@
 !endif
 
 !if ("$(GUI)"=="yes")
-DEFINES = $(DEFINES) -DFEAT_GUI_W32 -DFEAT_CLIPBOARD
+DEFINES = $(DEFINES) -DFEAT_GUI -DFEAT_CLIPBOARD
 !if ("$(DEBUG)"=="yes")
 TARGET = gvimd.exe
 !else