Although normally it only warns about it, under some circumstances,
aclocal can error out if this directory doesn't exist.
Reported-by: John Lumby <johnlumby@hotmail.com>
(Cherry picked from radeon commit 7b01c10137aba24c8f61dd9b2a19ea257ad24371)
Acked-by: Alex Deucher <alexander.deucher@amd.com>
