On Windows, we keep using our own implementations, because GCC
implements std::mutex and std::condition_variable with pthread
emulation, which is not a good choice.

Prepare the transition to std::mutex.

GetChromaprintCommand::DecodeStream() is called without holding the mutex.

Using this API function requires SDK level 23.

Fixes nullptr dereference when an exception gets thrown because there
is no ".eh_frame" section for unwinding.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/543

Instead of keeping all open outputs locked, let ClearTailChunk() stall
playback until MultipleOutputs::CheckPipe() has updated the MusicPipe.

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