* Fixed a bug which made frames go backwards with some displays (the
   video_decoder thread is now niced).
 * Made use of intf_WarnMsg() in the most needed places.
Also simplified the synchro algorithm and fixed a bug with non-(5/1) streams.
Fixed the optimizations in the configure stuff.
Warning : this release needs a full rebuild (make clean; make).
Warning : I might have broken the BeOS port, could somebody check ?