We can use the QT_LIB_FOO #defines instead of our own WITH_FOO ones to
determine if a library is available. Also, it doesn't currently make
sense to refer to libraries which are not part of qtbase here. We might
add that in the future, but QtScript (being deprecated) is probably not
the first one we should add.
Change-Id: I7f2397ca5499ba6003088478161182e960e815fb
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>