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/beval.h b/src/beval.h
index 21900c5..090c5fb 100644
--- a/src/beval.h
+++ b/src/beval.h
@@ -43,7 +43,7 @@
     int			y;
     unsigned int	state;		/* Button/Modifier key state */
 # else
-#  if !defined(FEAT_GUI_W32)
+#  if !defined(FEAT_GUI_MSWIN)
     Widget		target;		/* widget we are monitoring */
     Widget		balloonShell;
     Widget		balloonLabel;
@@ -63,7 +63,7 @@
     BeState		showState;	/* tells us whats currently going on */
 #  endif
 # endif
-# if !defined(FEAT_GUI_GTK) && !defined(FEAT_GUI_W32)
+# if !defined(FEAT_GUI_GTK) && !defined(FEAT_GUI_MSWIN)
     Dimension		screen_width;	/* screen width in pixels */
     Dimension		screen_height;	/* screen height in pixels */
 # endif
@@ -76,7 +76,7 @@
     int			*vts;		// vartabstop setting for this buffer
 #endif
     char_u		*msg;
-#ifdef FEAT_GUI_W32
+#ifdef FEAT_GUI_MSWIN
     void		*tofree;
 #endif
 } BalloonEval;