* Decouple Video4Linux2 support from Video4Linux as existence of
v4l2 on a system does not imply support for v4l.
* Don't use V4L's struct video_window in V4L2 code.
* Removed __USE_GNU as comment says:
/* support for MJPEG is only available with libjpeg and gcc,
because it's use libjepg and fmemopen()
so replace with test for fmemopen() if found necessary.
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