LU-11394 build: Remove SPL requirements

LU-11394 build: Remove SPL requirements

Because ZFS and SPL are version locked, and ZFS has explicit
requirements for SPL, remove lustre's SPL requirements.

lbuild: Make building spl optional when version is changed.

Lustre-change: https://review.whamcloud.com/33743
Lustre-commit: d61103d329bad5b92c951c8ad06a4f1b431759aa

Test-Parameters: trivial
Test-Parameters: fstype=zfs ostcount=2
Signed-off-by: Nathaniel Clark <nclark@whamcloud.com>
Change-Id: Iedba8d4047ba1fa852a2f99db2cd1b6caff33326
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/34653
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>