Log Message:
musicpd: update to 0.20.15.

ver 0.20.15 (2018/01/05)
* queue: fix crash after seek failure
* resampler
  - soxr: clear internal state after manual song change
* state file
  - make mount point restore errors non-fatal
  - fix crash when restoring mounts with incompatible database plugin
* Android
  - build without Ant
  - fix for SIGSYS crash

ver 0.20.14 (2018/01/01)
* database
  - simple: fix file corruption in the presence of mount points
* archive
  - bz2: fix deadlock
  - reduce lock contention, fixing lots of xrun problems
* fix Solaris build failure

ver 0.20.13 (2017/12/18)
* output
  - osx: set up ring buffer to hold at least 100ms
* mixer
  - alsa: fix rounding errors
* database
  - simple: don't purge mount points on update/rescan
  - simple: fix "mount" bug caused by bad compiler optimization
  - simple: fix "lsinfo" into mount points
  - upnp: work around libupnp 1.6.24 API breakage
* queue: fix spuriously misplaced prioritized songs
* save and restore mountpoints within the state file
* include Windows cross-build script in source tarball
* fix Windows build failures