Modified-by and Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

No need to invoke vlc-config to expand a shell variable.

There should be more V4L2-based plugins later, e.g. FM RX, FM TX, VOUT.

(GL/gl.h ought to be present if GL/glext.h is)

AC_CHECK_HEADERS does not work the way this change assumes. This commit
did not fix the stated problem.

This reverts commit 4e33295f.

We do not need to care which functions are in the math library. We only
need to care whether libm actually exists and can be linked against.

And by the way, lets define @LIBM@ for makefiles.

These days the extra vlc revision is an integer (1.1.10.1), not a
letter like in 0.8.6f days
Make PACKAGE_VERSION_* be integer, we don't need them to be strings
PACKAGE_VERSION_EXTRA now represents only this number, and is set to
0 when not used (1.1.11.0 represents 1.1.11)

Add PACKAGE_VERSION_DEV (== "git" or "rc*"), to be sure development
versions are less recent than released versions.
This string is never displayed, it is only tested for nullity

The file format on our servers can use X.Y.Z or X.Y.Z.E

Also implement correct run-time POSIX clock selection.