[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 #3 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
Well, the versions with two underscores at the beginning and two underscores at
the end seem to always be present, if this is what is intended. However even
that is not completely clear. For GNU/Linux, one has both __linux__ and
__gnu_linux__, but it is not described what they mean exactly and what are the
differences.