configure: check for Python Avahi module
configure: check for Python Avahi module

Previously we'd try to run the tests even if the Avahi Python module
wasn't available.

freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut