Jon Turney authored Apr 28, 2019 and Adam Jackson committed Apr 30, 2019

Currently, this can error if dri.pc isn't found, as we can't then get
the value of pkgconfig variable from it:

include/meson.build:199:10: ERROR:  'dri' is not a pkgconfig dependency

I think we need DRI_DRIVER_PATH (only) when building GLX, even if dri2/3
isn't enabled, so we know where to load swrast_dri.so from.

(For autotools, configure.ac directly calls `pkg-config
--variable=dridriverdir dri`, the backticks swallowing any error,
causing the value of this define to be empty if dri.pc isn't present)