LU-12745 build: Account for optional SPL for ZFS 0.8+

LU-12745 build: Account for optional SPL for ZFS 0.8+

With ZFS 0.8.0 and later, SPL is not longer present.
Some zfs packages provide vestigial spl package contents, but zfs-dkms
does not.  This makes testing SPL directories optional depending on
version of ZFS, this also accounts for the new location of the spl
include directory under the zfs include directory.

Lustre-change: https://review.whamcloud.com/36161
Lustre-commit: a245dde23a9fdbdff7d09a783bcbe3349f68a908

Test-Parameters: trivial
Signed-off-by: Nathaniel Clark <nclark@whamcloud.com>
Change-Id: I8afcff079f25543a3c86df0c404146a859b226aa
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36408
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>