Make use of dri2proto_CFLAGS when building.

Signed-off-by: James Le Cuirot <chewi@aura-online.co.uk>
Reviewed-by: Robert Morell <rmorell@nvidia.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>