package/x11r7/xdriver_xf86-video-ati: Add optional support for glamor

xserver_xorg-server depends on libepoxy for glamor support, therefore we
use this package as an indicator whether glamor support is present.

Fixes https://bugs.busybox.net/show_bug.cgi?id=8171



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>