contribs: force --target, --build and --host whether cross compiling or not to get around some configure script bugs

Doesn't lock the playlist if unneeded

* Quick fix when transcoding and resampling mono audio tracks.
* The transcode module should be reviewed: all the the fields in p
_enc->fmt_in are not set, it seems 

Comment unusued artist & album musicbrainz ids

Thanks to Daniel Stränger

Detect audio and video on CD-R and DVD-R mediums

Still not functionnal and ugly code.

* + a const fix

* configure.ac: when we use the packed attribute we really mean it so don't output useless compilation warnings.