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.
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