configure.ac: loosen --enable-glvnd check to honour egl

Currently we error out when building GLVND w/o GLX.

That was the original premice before we had EGL. As the commit says,
that error should be reworked to honour both - do so.

v2: Drop noop *);; (Eric)

Reported-by: Lukas Rusak <lorusak@gmail.com>
Fixes: ce562f9e ("EGL: Implement the libglvnd interface for EGL (v3)")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Tested-by: Lukas Rusak <lorusak@gmail.com> (v1)
(cherry picked from commit b4967561)