I'm pretty sure a project as widely used as FFmpeg can't just choose a single compiler (and you'd actually have to choose a single compiler version since for instance new versions of GCC can introduce new warnings).

On top of that, sometimes warnings come from parts of the code you don't control, like a scanner generated by flex.

I think it's better to have a strong policy of not introducing warnings and fixing them whenever a particular compiler is foud to emit them, but having -Werror in the default Makefile can quickly inundate the project's mailing list/IRC channel with "doesn't compile" complaints from people with exotic compilers.