I'm testing this out now and I find that defaulting to the value returned for `systemduserunitdir` by `pkg-config` means that even when configured with a `--prefix`, `make install` tries to install into the system directory. This in turn means that make distcheck fails because it does not have permission to install to `/usr/lib/systemd/user`. I'm wondering what is the best way to fix that. Can we perhaps just default to `@libdir@/systemd/user` (which will correctly prepend the prefix) and let the user specify a different directory using `--with-systemduserunitdir=X`?