Respect install prefix when installing systemd service file.

[vogr]

Fixes #11.

When using relative paths in the install directive (docs):

If a relative path is given it is interpreted relative to the value of the CMAKE_INSTALL_PREFIX variable.

When no prefix is given, the default is /usr/local, so the service file would end up at /usr/local/lib/systemd/system. This is the right directory for files installed by the administrator according to the systemd docs.

This doesn't work for local installs because systemd expects the service file in $HOME/.local/share/systemd/user and not $HOME/.local/lib/systemd/system ... But since the daemon expects to run as root anyway, this is not a big problem here.

Thank you in advance!

[vogr]

I realized that /usr/bin/ibarr was hard-coded in the service file: I made a second commit to use the CMAKE_INSTALL_PREFIX there too. This works even for installs with a staging directory as described in the issue.