Processing configuration for gn when building qpdfcore module
will require optional libraries resolved. Some libs are not needed,
but this requires extra patching of gn build configs; so
to keep things simple, just detect all the libs beforehand.

Unfortunately using this for main configure goes bonkers.
The issue here is that main configure resets features in global scope,
which can break features coming from qtbase and results in
bogus configurations in certain cases. Therefore move current
configuration to buildtools and make everything else depend on this.

Task-number: QTBUG-75840
Task-number: QTBUG-76606
Change-Id: Id87f5ba017541a2d5a2836510c6cabc5a19f92c6
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>