systemd: Use automake tools to install unit files

systemd: Use automake tools to install unit files

If systemd is enabled we install unit files to $libdir/systemd/system
(or the path specified by SYSTEMD_UNIT_DIR).
The unit files are generated on the fly with matching $sbindir.

Signed-off-by: Christian Hesse <mail@eworm.de>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20170124143947.27385-1-list@eworm.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13940.html
Signed-off-by: David Sommerseth <davids@openvpn.net>