Updated musicpd to 0.19.15.

Switch to ffmpeg3.

ver 0.19.15 (2016/04/30)
* decoder
  - ffmpeg: support FFmpeg 3.0
  - ffmpeg: use as fallback instead of "mad" if no plugin matches
  - opus: support bigger OpusTags packets
* fix more build failures on non-glibc builds due to constexpr Mutex
* fix build failure due to missing include
* fix unit test on Alpha

(wiz)