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)
c2feeca1b0