It is a good practice to install files relative to our own variables
https://www.bassi.io/articles/2018/03/15/pkg-config-and-paths/
and it is required on systems like NixOS.
Unfortunately, systemd allows overriding the root prefix,
see also 1c2c7c6cb3,
so we cannot just do that.
Let's at least make the systemd installation prefix overridable.
This will also allow us to drop `systemdsystemunitdir` option since
systemd hardcodes it to `${rootprefix}/lib/systemd/system`.
463db5162f
