qbs files: fix OS X checks

Change-Id: I618c1ad4bf328492403b48c8e398abd538f74787
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>