old-build: fix iconv check on openbsd
build: add custom -I/-L flags for the BSDs

Apparently this is needed for stuff like iconv.
ao_coreaudio: simplify ch label to speaker id conversion

Previous code was using the values of the AudioChannelLabel enum directly to
create the channel bitmap. While this was quite smart it was pretty unreadable
and fragile (what if Apple changes the values of those enums?).

Change it to use a 'dumb' conversion table.
ao_coreaudio: allow to use exclusive mode with lpcm
ao_coreaudio: redo stream/format selection