The current method to build against DKMS installs of ZFS and SPL
assumes directories like this:

   /var/lib/dkms/spl/${VERSION}/build
   /var/lib/dkms/zfs/${VERSION}/build

Unfortunately, that "build" directory was really just a temporary
step in the DKMS build process. Older versions of DKMS forgot to
clean up that directory, but in RHEL7 at least, DKMS no longer
leaves that around.

Instead we need to look in these places for source code:

  /var/lib/dkms/spl/${VERSION}/source
  /var/lib/dkms/zfs/${VERSION}/source

and in these places for the per-kernel object info:

  /var/lib/dkms/spl/${VERSION}/${LINUXRELEASE}/${target_cpu}
  /var/lib/dkms/zfs/${VERSION}/${LINUXRELEASE}/${target_cpu}

The changes in this patch update the Lustre build system to
look in those corrected directories for DKMS builds of SPL
and ZFS.