systemd generators: Install depending on location for systemd unit files

While it is possible to configure the systemd unit files directory on
configure time with --with-systemd=unit-dir-path, this path is not taken
into account for installing the systemd unit generators, as they are
installed unconditionally in /usr/lib/systemd/system-generators.

Signed-off-by: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Steve Dickson <steved@redhat.com>