Make XF86VIDMODE extension optional

Make XF86VIDMODE extension optional

Code in glx/glxcmds.c which uses the XF86VIDMODE extension is already guarded.  Also use
that guard to control inclusion of the xf86vmode.h header, and only enable that guard if the
XF86VIDMODE extension is found by pkgconfig.

This changes the behaviour on platforms which XF86VIDMODE exists, in that XF86VIDMODE used to
be mandatory, but is now optional.

Presumably other build systems are already arranging for -DXF86VIDMODE to be supplied to the
complier when glxcmds.c is compiled, so are not affected by this change

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>