Make sure that the log callback is installed.

Use the duration of the stream we're actually decoding - not the
"global" attribute AVFormatContext::duration which may differ.

Fixes regression by commit 7c8038e2

Allow multiple targets to share a library directory tree.

setvbuf() does not seem to have an effect on Windows.

Make sure everything gets logged right away.  No delays because
stdio's buffer is not yet full.