[Bug preprocessor/69473] system identification macros not documented

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69473

--- Comment #5 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to joseph@codesourcery.com from comment #4)
> __linux__ and __linux should always be defined for systems with the Linux 
> kernel (plain linux only in non-ISO modes);

As said in bug 28314 (resolved as wontfix), __linux is not defined on
Linux/PowerPC in C99 mode (-std=c99).