updated for version 7.3.500

Problem:    Ming makefile unconditionally sets WINVER.
Solution:   Only defined when not already defined. (Yasuhiro Matsumoto)

src/version.c

@@ -714,6 +714,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    500,
 /**/
     499,
 /**/