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_bc5.mak b/src/Make_bc5.mak
index 04f7885..3dd0421 100644
--- a/src/Make_bc5.mak
+++ b/src/Make_bc5.mak
@@ -405,7 +405,7 @@
 !endif
 
 !if ("$(GUI)"=="yes")
-DEFINES = $(DEFINES) -DFEAT_GUI -DFEAT_CLIPBOARD
+DEFINES = $(DEFINES) -DFEAT_GUI_MSWIN -DFEAT_CLIPBOARD
 !if ("$(DEBUG)"=="yes")
 TARGET = gvimd.exe
 !else