mpd version 0.17.3
Merge tag 'release-0.17.3'
audio_config, volume, mixer_all: include cleanup
volume: convert to C++
mixer_all: convert to C++
decoder_api.h, ...: add "extern C"
filter/replay_gain: convert to C++
replay_gain_info, ...: use cmath instead of math.h in C++ mode

Fixes build problems with mingw32.
thread/Mutex: don't use std::mutex

Use a custom pthread_mutex_t wrapper because std::mutex adds overhead.
DatabaseLock: use the Mutex class instead of GLib's GMutex
tag: convert to C++
TagPool: use the Mutex class instead of GStaticMutex
Client: add Partition reference attribute

playlist and player_control are deprecated.
DatabaseQueue: pass DatabaseSelection to Add...()
Playlist: convert functions to methods
Queue: move queue_shuffle_order_range() into the class
Queue: move queue_shuffle_order_first() into the class
util/LazyRandomEngine: lazy wrapper for std::mt19937
Queue: use C++ random instead of GLib's GRand
Queue: don't include glib.h