Use C99 printf statements in mingw

Otherwise, format specifiers such as %llu will not work on XP or
earlier.
This bug was introduced with 51ed9ec9.
http://sourceforge.net/apps/trac/mingw-w64/wiki/gnu%20printf

src/util.cpp

@@ -336,11 +336,7 @@ string vstrprintf(const char *format, va_list ap)
     {
         va_list arg_ptr;
         va_copy(arg_ptr, ap);
-#ifdef WIN32
-        ret = _vsnprintf(p, limit, format, arg_ptr);
-#else
         ret = vsnprintf(p, limit, format, arg_ptr);
-#endif
         va_end(arg_ptr);
         if (ret >= 0 && ret < limit)
             break;