http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58016
--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot
com> ---
__STDC_VERSION__ describes *intent* of command-line options (as regards
differences between standard versions, to the extent that those are
implemented). This is the same principle that has been documented for
__STDC__ since at least GCC 2.0.
"Sometimes people say that defining @code{__STDC__} in a compiler that
does not completely conform to the ANSI C standard somehow violates the
standard. This is illogical. The standard is a standard for compilers
that are supposed to conform. It says nothing about what any other
compilers should do. Whatever the ANSI C standard says is relevant to
the design of plain @samp{gcc} without @samp{-ansi} only for pragmatic
reasons, not as a requirement."
(quoted from the GCC 2.0 manual).
As a pragmatic matter, it's useful for users of standards modes that are
incomplete to be able to tell which of those modes is in use, and
__STDC_VERSION__ is the natural macro to define to distinguish between
them. gcc -std=c11 is a compiler explicitly claimed not to conform.
