Detect V4L availability

Do not build related stuff if not found.
Makes GStreamer support available on Hurd.

Task-number: QTBUG-39762
Change-Id: I1f70b6975e5bef99ab2441aac4d90508bc8b64bd
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>