glx: conditionaly build dri3 and present loader (v3)

glx: conditionaly build dri3 and present loader (v3)

This patch makes it possible to disable DRI3 if desired.

Tested with:

./configure --disable-dri3 --with-dri-drivers=i965 \
--with-gallium-drivers= --disable-vdpau --disable-egl \
--disable-gbm --disable-xvmc

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71397
Cc: 10.0 <mesa-stable@lists.freedesktop.org>