Patch mpd with floating planar audio support

Playing m4a files with faad2 will cause segmentation fault. If 'mp4ff' plugin is explicitly disabled, then m4a decoder from ffmpeg is preferred, which actually cannot play any m4a files from iTunes since fltp32 SampleFormat is not supported by mpd 0.17.3.
(See #15788.)

Though this patch has already been applied upstream, it seems that the development repository is currently performing migration to C++11, everything there looks like a mess.

The patch:
(https://github.com/bjaglin/mpd/commit/a84774fd46e4f3b6147bfd3d19ff4841bde8c98d)