CMake: Don't check the existence of GL files in the Qt5Gui package.

The GL libraries are optional, so they should not be added to the
DEPENDENT libraries of Qt5::Gui if not found.

Task-number: QTBUG-39859
Change-Id: Ib0e01da56a9fb5048a6ad8e0e9cede07fe0cf43b
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>