vtk: Use the Qt libraries as an argument instead of a boolean

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