v4l2: Add run-time environment to enable libv4l2

v4l2: Add run-time environment to enable libv4l2

The library has started preventing a lot of interesting use cases,
like CREATE_BUFS, DMABuf, usage of TRY_FMT. As the libv4l2 is totally
inactive and not maintained, we decided to disable it. As a convenience
we added a run-time environment that let you enable it for testing.

  GST_V4L2_USE_LIBV4L2=1

This of course only works if you have enabled libv4l2 at build time.