Previously it was not possible to disable gpu acceleration if every dependency was discovered by meson. Fix that by adding a new option. Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com> Reviewed-by: Milan Zamazal <mzamazal@redhat.com> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>