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/if_tcl.c b/src/if_tcl.c
index eb6684c..46378a2 100644
--- a/src/if_tcl.c
+++ b/src/if_tcl.c
@@ -160,7 +160,7 @@
 typedef int HANDLE;
 # endif
 
-# ifndef WIN3264
+# ifndef MSWIN
 #  include <dlfcn.h>
 #  define HANDLE void*
 #  define TCL_PROC void*
@@ -1741,11 +1741,11 @@
 #endif
 
 	Tcl_SetChannelOption(interp, ch1, "-buffering", "line");
-#ifdef WIN3264
+#ifdef MSWIN
 	Tcl_SetChannelOption(interp, ch1, "-translation", "lf");
 #endif
 	Tcl_SetChannelOption(interp, ch2, "-buffering", "line");
-#ifdef WIN3264
+#ifdef MSWIN
 	Tcl_SetChannelOption(interp, ch2, "-translation", "lf");
 #endif