These messages can be confusing, but they are not critical.

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

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.

This fixes the assertion failure due to calling
AudioOutputSource::Close() twice.

Fixes a crash bug with `always_on` outputs which occurs because the
`AudioOutputSource` still has a pointer to an outdated `MusicChunk`.

Fixes #415

With the new command, the decision to pause or close the output moves
into the output thread.

Prepare for adding another Cond attribute.

Fixes deadlock due to mutex double lock.

This will be used by filters which have internal buffers which need to
be flushed at the end, e.g. the "soxr" resampler.

This call cannot fail, but needs cleanup if others fail.  By moving it
to the end, we eliminate some cleanup.

Moved code from AudioOutputControl::InternalOpen2().