link to libFLAC/ogg statically for osx

Not sure if this is the best way to be doing this.  ATM we're dynamically linking to flac+ogg, which probably isn't what we want given that these aren't distributed with XBMC any longer, at least if we're going to be distributing the binary add-ons separately.

I take no credit for the .a magic - that's @wsnipex.

@notspiff any comments/suggestions most welcome.  The dynamic linking may well only be appropriate for lesser platforms?