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_lua.c b/src/if_lua.c
index e650e5e..12643f9 100644
--- a/src/if_lua.c
+++ b/src/if_lua.c
@@ -80,7 +80,7 @@
 
 #ifdef DYNAMIC_LUA
 
-#ifndef WIN3264
+#ifndef MSWIN
 # include <dlfcn.h>
 # define HANDLE void*
 # define load_dll(n) dlopen((n), RTLD_LAZY|RTLD_GLOBAL)