Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

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 AudioOutput struct (which is exposed to all plugins) should not be
aware that it's being controlled by another thread.

Obsoletes the same variable from AudioConfig.cxx.

Remove dependency on ReplayGain global variables.

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.

Auto-stop the IO thread in all error handlers.