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.
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