You can tell by "Mesa/configs/default" how old this is.  Your build system
really has to provide the DEFAULT_DRIVER_DIR, or other loaders will break.

v2: Move the bad (non-prefix-dependent) define to the SConscript to avoid
    breaking it.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com> (v1)
Reviewed-by: Emil Velikov <emil.velikov@collabora.com> (v1)