When librespot is built with Avahi turned on, it will crash if Avahi is
later not available at runtime.
This change avoids it crashing hard when Avahi is not available;
librespot will merely warn of the issue.
This affects some distribution packages too, where the maintainer might
prefer leaving Avahi support enabled, but many setups don't (or can't)
run Avahi.
Co-authored-by: Nick Steel <nick@nsteel.co.uk>