configure: remove legacy Altivec checks - should fix #9757
Audio decoder: output buffered decoded frames on EOF

If EOF happens during buffering, the buffered frames will be flushed
and never played back.
This should only be visible for very short files such as:
http://streams.videolan.org/issues/9195/73ms-VLC-KO.wav

Note, pulse will lose the audio for such short files, alsa and afile work better

Closes: #3152