build: Move systemd checks to lib/util

build: Move systemd checks to lib/util

Only lib/util uses the systemd library, so it makes sense to have the
checks there. This also removes the need for the ctdb build script to
specify an empty tag for the systemd library.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>