Log Message:
Updated musicpd to 0.19.18.

ver 0.19.18 (2016/08/05)
* decoder
  - ffmpeg: fix crash with older FFmpeg versions (< 3.0)
  - ffmpeg: log detailed error message
  - ffmpeg: support FFmpeg 3.1
  - sidplay: detect libsidplay2 with pkg-config
  - sidplay: log detailed error message
  - sidplay: read the "date" tag
  - sidplay: allow building with libsidplayfp instead of libsidplay2
* output
  - shout: recognize setting "encoder" instead of "encoding"
* fix memory leak after stream failure
* fix build failure with Boost 1.61
* require gcc 4.7 or newer