Add support for video4linux devices by:
- adding a build dependency on v4l_compat;
- fixing the paths to locate videodev.h in configure.in,
and disabling the (incorrect) check for V4L2;
- correct the paths in grabber-video4linux.cpp, and also add
support for the read() method in addition to mmap.
On the mailing list there was a suggestion to remove the
hardwired path in favour of a variable ( %%LOCALBASE%% or so)
based one. But this configure.in is full of hardwired paths,
so fixing one doesn't help at all, and the full fix would take
more time and knowledge that i have.