mesa-demos: fix detection of GLX headers

mesa-demos: fix detection of GLX headers

Backport a patch from upstream to fix the build when glx.pc isn't found,
but the glx headers are available. As libglvnd is the only provider of
glx.pc and by default we build without libglvnd, this is common.

This solves the problem where mesa-demo is missing the GLX demos, such
as glxgears.

[ YOCTO #15825 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>