As Qt applications using OpenGL are linked against these libs, merging
them into QtANGLE by default (780105f9

)
was a binary incompatible change. This change restores the default
behavior to the one before given change.

If the user wants the libraries to be merged, he can pass
combined-angle-lib to configure.

Task-number: QTBUG-60373
Change-Id: Iedbd3f2ce9284fdde924cfae8d915d6d5fef00db
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>