We need to substitute root_sbindir and root_libdir even for the case
where we don't have the different from the default prefix, otherwise
xfsprogs won't build for that case with rpath errors, and wouldn't
install correctly either.
Signed-off-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Alex Elder <aelder@sgi.com> Reported-by: Christian Kujau <lists@nerdbynature.de> Tested-by: Christian Kujau <lists@nerdbynature.de>