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>