Fix OSX audio problems

[cjcliffe]

Having issues using PortAudio and LibAO in a thread -- OSX won't give them enough priority to be functional? Always under-running the buffer in a thread even with blocking write and high priority audio I/O thread. I/Q thread and demod thread are already pushing way more data than audio without a hiccup...

Current options:

1. Keep monkeying with pthread native flags and settings, not sure if it's the threading at this point; looks like audio lib internals
2. Write custom audio layer for OSX and figure out the issue myself
3. Give https://github.com/thestk/rtaudio by @thestk a try and see if it solves anything.
4. Switch to pthreads entirely? Would prefer to keep std::thread and don't think this would help as std::thread is apparently already using pthreads on OSX

[cjcliffe]

Going to close this as RtAudio is the answer -- Now it's just a matter of balancing out the threads IMHO