Added option to link shairport with avahi, rather than calling it externally.

Added the option to link "shairport" binary with avahi, rather than calling the external "avahi-publish-service" command. This is cleaner, and it means the service broadcast is reliably removed on crash or exit. To enable: uncomment the line setting LINKAVAHI in the Makefile. Requires the libavahi-client-dev package on Ubuntu/Debian to compile.