[Bug preprocessor/69473] system identification macros not documented

[joseph at codesourcery dot com]

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

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
__linux__ and __linux should always be defined for systems with the Linux 
kernel (plain linux only in non-ISO modes); __gnu_linux__ for systems with 
the Linux kernel and glibc (the nearest thing to the concept of GNU/Linux 
systems available to GCC).  That should work for all systems using 
config/linux.h, which covers most but not all targets using the Linux 
kernel.