We require glx version 1.3 which is where fbconfig is first defined.
Also make use of the configure test and report the glx status.
GLX support should always take precedence when compiling the
xcb backend
Change-Id: Ie46834210bf5cd2ac4006ff08379e0d3434ffa2b
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
