Strip the -L dirs when processing GL dirs for the CMake files.

The generated cmake files expect only the names of libraries, so the
existence of directories causes erroneous fatal errors when attempting
to use Qt5Gui, if pkg-config returns a -L entry from

 pkg-config --libs egl

Change-Id: Iec50b4be68ab643c3c02abce2435a98e69955138
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>