drirc is not loaded when mesa doesn't use /usr/share for datadir

pressure-vessel copies the drirc files to /usr/share/drirc.d, but this is only correct if (the about-to-be-used version of) mesa was configured to use /usr/share as its datadir. For all other cases, this is broken, and mesa will fail to load any drirc files.

To fix this, something like this would be needed:

• check pkg-config --variable=datadir dri and copy files to this location if the value isn't empty
• if the above fails, use $(pkg-config --variable=prefix dri)/share
• only if both of the above fail use a hardcoded /usr/share