LU-15956 gss: allow build without ssk

LU-15956 gss: allow build without ssk

The GSS part of Lustre should be able to build without SSK, in case
some SSK requirements are not met at configure time.

Lustre-change: https://review.whamcloud.com/47681
Lustre-commit: 2e061e88a44db70e4446be567243a38a76ed5370

Test-Parameters: trivial
Test-Parameters: testgroup=review-dne-selinux-ssk-part-1
Test-Parameters: testgroup=review-dne-selinux-ssk-part-2
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ieacbcb5db77fcc12cc13579785e640857ce7fb02
Reviewed-on: https://review.whamcloud.com/47781
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>