configure: determine the systemd unit directory from pkg-config

Ask systemd instead of hardcoding the path. While this is a bit nicer,
it should have precisely zero effect as the discovered path should be
the same as we were hard-coding.

We default to placing the unit file under the same $prefix as the user is
installing into. This seems to be the correct thing to do if the user is
installing to /usr/local (according to systemd-path(1),
/usr/local/lib/systemd/system is all right), but can install the unit
file into wrong path if the user chooses to install into some obscure
location. I guess it's their responsibility in the end.
51 jobs for lr/ac-systemd-unit-path in 64 minutes and 28 seconds (queued for 13 seconds)