this is much more elegant than the so far propagated !isEmpty(QT.foo.name).
also replace feature-specific tests (no-gui and no-widgets) and the
obsolete contains(QT_CONFIG, foo) syntax.

Change-Id: Ia4b3c8febcabf9eeca67b1f9173a523820b1038b
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Reviewed-by: Tasuku Suzuki <stasuku@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>