Recognize newer libmng versions in config test

libmng 2.0.x has been released and is compatible and usable, but since
it no longer provides a VERSION_MAJOR macro, the config test would fail.

Task-number: QTBUG-34894
Change-Id: I106aa258de0851af01d1bb016c2971dd8e30fd24
Reviewed-by: Liang Qi <liang.qi@digia.com>

config.tests/libmng/libmng.cpp

@@ -46,8 +46,10 @@ int main(int, char **)
     mng_handle hMNG;
     mng_cleanup(&hMNG);
 
+#if defined(MNG_VERSION_MAJOR)
 #if MNG_VERSION_MAJOR < 1 || (MNG_VERSION_MAJOR == 1 && MNG_VERSION_MINOR == 0 && MNG_VERSION_RELEASE < 9)
 #error System libmng version is less than 1.0.9; using built-in version instead.
+#endif
 #endif
 
     return 0;