Optional alsa implementation for hairport

Hi to all,
i'm proposing this fix for compile (optionally) shairport with alsa instead of ao
I'm working on an airplay server over a stb and i use hairport for itunes protocol but unluckily my system doesn't provide libao. So i made this patch for compile it with alsa instead of ao. I think this could be of interest for anyone want compile it on an embedded device.

Thanks :)

Sandro