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/screen.c b/src/screen.c
index d06cc3f..4973b51 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -7974,7 +7974,7 @@
 
     screen_attr = attr;
     if (full_screen
-#ifdef WIN3264
+#ifdef MSWIN
 		    && termcap_active
 #endif
 				       )
@@ -8081,7 +8081,7 @@
     int	    do_ME = FALSE;	    /* output T_ME code */
 
     if (screen_attr != 0
-#ifdef WIN3264
+#ifdef MSWIN
 			&& termcap_active
 #endif
 					   )