When fluidsynth is run as a service using systemd, make sure
the service is considered started only when it is ready to process events.
In order to do so:
- Add an optional runtime dependency to libsystemd to the fluidsynth executable
- Change the systemd service type to "notify"
- Have fluidsynth notify systemd that the service is started after the server is started
- Have fluidsynth notify systemd that the service is stopping after joining the server thread