build: use as-needed linker flag also on OpenBSD

build: use as-needed linker flag also on OpenBSD

OpenBSD is unusable with binaries with many superfluous libs linked in.
samba-tool start times of 250 seconds without as-needed vs. 1.4 seconds with
as-needed.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11355

Signed-off-by: Bjoern Jacke <bj@sernet.de>
Reviewed-by: Ralph Böhme <rb@sernet.de>
Autobuild-User(master): Björn Jacke <bj@sernet.de>
Autobuild-Date(master): Fri Sep 11 03:37:17 CEST 2015 on sn-devel-104

(cherry picked from commit b0f41c07ffe8600433c20a038b1612c04ed29e89)