Fix mir detection

The old check would have never triggered unless you compiled
with CONFIG+=mir by hand.

Change-Id: I1d418dce09adaedbdff366dfeb8f4036cac0f075
Reviewed-by: Yoann Lopes <yoann.lopes@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>