opencv: rework V4L/libv4l support

opencv: rework V4L/libv4l support

Starting with the 2.4.7 release, for V4L support, OpenCV does not
requires a libv4l dependency for its V4L support.

So, it now provides 2 distincts knobs for this: WITH_V4L and a new
one: WITH_LIBV4L.

This patch takes advantage of this new knob and libv4l support is
now automatically enabled only if the libv4l package is enabled.

In OpenCV, the logic behind this is:
- using libv4l if available,
- otherwise, use v4l2 or v4l API from the kernel if available.

[Thomas: remove gratuituous reformating, as suggested by Yann.]

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>