Build: configure: make systemd detection akin to + before Upstart one

Mirroring Upstart treatment includes running systemctl as a systemd
detection fallback (mainly for chroot-based build systems), with
the logic requiring neither systemd nor its development files to be
present at the time of building pacemaker when desired path to place
unit files defined through systemdunitdir environmental variable
being preserved.

The new ordering reflects both the natural evolutionary timing and the
(implied) order in which the support should be tried (newest to oldest).