Fix breakage from commit bea5681f

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

The old struct config_param remains only for top-level string options.

Since version 2.32, g_thread_init() is deprecated and a no-op.  Let's
upgrade that (optional) dependency so we can remove all those
g_thread_init() calls.

Use a listener interface instead of GlobalEvents.

Passing a void pointer is unsafe.

Avoid GLib.

Prepare for adding state.

Replaces GLib's GError.

Deprecated.