package/nvidia-driver: don't install gl.pc when not providing libgl

package/nvidia-driver: don't install gl.pc when not providing libgl

When nvidia-driver does not provide libgl (i.e. when it does not install
the X.org driver), it should also not install gl.pc, otherwise, packages
that depend on pkg-config will mis-detect presence of GL and fail to
build, like vlc:

    http://autobuild.buildroot.org/results/724/724fce8ce51c2c0578192b1369a1cfcea3d72638/

So, only install gl.pc when we install the X.org driver. Similarly, we
do not need to install libGL.la either.

Reported-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>