https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86940
--- Comment #4 from Aritz Erkiaga <aerkiaga6 at gmail dot com> --- Ok, then this is not a bug. Maybe a feature request? Well, actually I think it would be safe for the compiler to assume that libc (and possibly other external libraries) don't have access to variables defined in the source file being compiled. Of course, they could do whatever they like, but the possibility is not worth a warning, as I see it.
