[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 #6 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Vincent Lefèvre from comment #5)
> As said in bug 28314 (resolved as wontfix), __linux is not defined on
> Linux/PowerPC in C99 mode (-std=c99).

Just in case this isn't clear (because bug 28314 is very old), the behavior has
not changed: this is still the case with gcc (GCC) 4.9.2 20141101 (Red Hat
4.9.2-1), as tested on gcc112.fsffrance.org:

[vinc17@gcc2-power8 ~]$ gcc -std=c99 -dM -E -xc /dev/null | grep linux
#define __linux__ 1
#define __gnu_linux__ 1