Avahi, including the C client library, is licensed under the LGPL.
On my system, (and I believe everywhere)
libavahi-client.so.3 links to
libdbus-1.so.3. DBus is licensed under a dual license, GPL OR AFL.
The AFL is documented as being incompatible with the GPL.
This answer explains that software licensed under the LGPL is required to be compatible with the GPL, thus Avahi cannot link with DBus under the terms of the AFL.
If Avahi cannot link to DBus under the terms of the AFL, it must be linking to it under the terms of the GPL (this is how dual-licenseing works).
But, if Avahi is linking to DBus under the terms of the GPL, it must be GPL itself.
What I have written above is my understanding, and I am looking for confirmation if my understanding is correct or why I am wrong.