glx: link loader util lib only when building with dri3

glx: link loader util lib only when building with dri3

Otherwise we pull libudev as a dependency and crash
games/programs that ship their own version of libudev.

Either way we should link the loader lib only when needed.
This fixes a regression caused by
commit eac776cf779b705cbfb8d41812f1d171fb09c76f
Author: Emil Velikov <emil.l.velikov@gmail.com>
Date:   Sat Jan 11 02:24:43 2014 +0000

    glx: use the loader util lib

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73854
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>