ffmpeg, aconv, Libav: what's going on ?

If you are wondering why you get a strange message when using ffmpeg: here is the answer.

You will find plenty of references in English on the web (and a couple in the french version of this article).

There's nothing more in English:

•     if you want an English translation, ask for it in the comments.
•     or switch to French by clicking on the command in the top right corner.