Regression test portability: arequal-checksum

Emulate GNU libc extension FTW_SKIP_SUBTREE for system that lack it.
FTW_ACTIONRETVAL is another GNU libc extension we just ignore now.

BUG: 764655
Change-Id: I25d9641055a30fe72b4e248f51b53b2a3ba637e9
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8427
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>