better preprocessor check for recent MSVC versions

lib/config-win32.h

@@ -352,7 +352,7 @@
 /* Undef keyword 'const' if it does not work.  */
 /* #undef const */
 
-#if _MSC_VER > 1310
+#if defined(_MSC_VER) && (_MSC_VER > 1310)
 /* MSVC 2003 has gmtime_r */
 #define HAVE_GMTIME_R
 #endif