Let Apple Clang 5.0 also have -Werror support

Change-Id: I3abc83617c8f3426e4f5920052281fe13e4e3edc
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>