Find avahi-common library

When building zeroconf_avahi_suite on Fedora zeroconf_avahi_demos fails. Specifically, official_browse_example and official_publish_example need to link against avahi-common. FindAvahi.cmake seemed like the appropriate place to search for the library. I've tested this change on both Fedora 18 and Ubuntu 12.04 and avahi-common is found and everything compiles and links correctly on both.