I have taken a look at the avahi package. You will surely agree that a package modifying its own debian/changelog file during the build would be an oddity which surely would be flagged as bad practice (if not directly a bug).
I have seen packages using debian/control.in and sometimes debian/copyright.in, but the changelog is supposed to be "final" for a given package upload, not to be modified during the build. What avahi does is similar, but it modifies (by creating it) an upstream changelog when there was none. Please reassign and retitle, I'm sure you will be able to explain this to the avahi maintainers. Thanks.
