Only support systemd: transport if we have systemd libraries

This means we can drop our convenience copy of sd-daemon.[ch]. We're
checking for libsd-daemon anyway, to support journald and logind
integration.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=71818
Reviewed-by: Lennart Poettering <lennart@poettering.net>
Fix error message.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=73887
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>