LU-13549 build: fix zfs/spl config checks

LU-13549 build: fix zfs/spl config checks

zfs/spl config checks should only proceed to variable substitution in
case system supports it. Otherwise, it would end up adding 'Not found'
in list of include dirs/libs.

Test-Parameters: trivial testgroup=review-zfs
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Ia76a67e42cc6a2f116013c142b6c0b2143838548
Reviewed-on: https://review.whamcloud.com/38592
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shuichi Ihara <sihara@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>