We need pthreads and since WebAudio does not support linking non-pthread
and pthread-enabled object files, we have to make this public.

This should hopefully ensure that Emscripten is actually used

While we could try multiplexing streams into a single audio context,
we can only set one sink per context. Both therefore, and because the
start/stop semantics seem to map nicely to AudioContext's
resume/suspend, we will create a context per stream.

This synchronous mode uses `emscripten_thread_sleep` instead of
`emscripten_sleep` and is intended to run from a separate thread where
blocking is allowed.

In its current state, this seems to depend on

    WebAudio/web-audio-api#2423

The alternative would be to proxy audio context-related calls to the
main thread.