Enable DRI3 in xorg-server

**TEST needed**. I tested on my NixOS Thinkpad with Optimus nvidia and intel
cards.  Testing in other architectures is needed.

This patch also improves the regular expressions that scan the build
configuration files to enable the building of a binary utility in
xf86-video-intel: intel-virtual-output. This utility is useful for some
Optimus laptops

DRI3 was added to xorg-server propagatedBuildInputs so now it's inherited
by several projects that can use it.