mesa: don't install GLX files if GLX is not built

Cc: "11.2 12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Akihiko Odaki <akihiko.odaki.4i@stu.hosei.ac.jp>
[Emil Velikov: Drop guards around dri_interface.h, add stable tag]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
2 files changed