Make libsystemd optional again

As much as I'd like to stop supporting non-systemd Linux systems, some
people get very upset by the suggestion that systemd is mandatory on
Linux, and it isn't portable to non-Linux in any case.

Signed-off-by: Simon McVittie <smcv@debian.org>
Bug: https://bugzilla.gnome.org/show_bug.cgi?id=788470


Reviewed-by: Emmanuele Bassi <ebassi@gnome.org>