util: Add detection of libunwind

The current detection doesn't seem to work, so libunwind doesn't seem
to be used.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Volker Lendecke <vl@samba.org>

Autobuild-User(master): Martin Schwenke <martins@samba.org>
Autobuild-Date(master): Tue Jan 21 08:12:17 UTC 2020 on sn-devel-184