correct cleaning of TZ

add a check of the version Visual C

mysys/my_init.c

@@ -242,8 +242,13 @@ static void my_win_init(void)
 
   setlocale(LC_CTYPE, "");             /* To get right sortorder */
 
-  /* Clear the OS system variable TZ and avoid the 100% CPU usage */
+#if defined(_MSC_VER) && (_MSC_VER < 1300)
+  /* 
+    Clear the OS system variable TZ and avoid the 100% CPU usage
+    Only for old versions of Visual C++
+  */
   _putenv( "TZ=" ); 
+#endif  
   _tzset();
 
   /* apre la chiave HKEY_LOCAL_MACHINES\software\MySQL */