To prevent multiple Qt libraries when developing with a custom one, the Qt
support can now be activated by directly supplying the Qt libraries as an
argument (qtLib).
qtSDK and qtFull users/developers now just have to define an override such
as the following one in order to use it inside their development
environment:
vtk.override { qtLib = qt4SDK; };
The previous behavior is still the same for vtk and vtkWithQt4 end-users.
Change-Id: I517762d4ff7de46d32cc46e6e725fd62737caa52
736b295bc5