build: Add libva dependency fallback.
After https://github.com/intel/libva/pull/541 is merged, it would be possible to have libva as subproject.
Then libva can be subproject and used by either gstreamer-vaapi, va and msdk in gst-plugins-bad.
After https://github.com/intel/libva/pull/541 is merged, it would be possible to have libva as subproject.
Then libva can be subproject and used by either gstreamer-vaapi, va and msdk in gst-plugins-bad.