https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85995
--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot
com> ---
See trouble.texi, "Non-bugs" / "Certain Changes We Don't Want to Make",
"Undefining @code{__STDC__} when @option{-ansi} is not used." (and the
description of handling of base standards in invoke.texi). This is as
documented since GCC 2.0.
