[Bug preprocessor/64220] New: gcc preprocessor defines outside of the reserved namespace: unix linux AVR

["cameron at tacklind dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64220

            Bug ID: 64220
           Summary: gcc preprocessor defines outside of the reserved
                    namespace: unix linux AVR
           Product: gcc
           Version: 4.9.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cameron at tacklind dot com

The preprocessor defines names outside of the reserved namespace.

Essentially, if I'm not mistaken, the following command should output nothing
# gcc -dM -E - < /dev/null | grep -v '^#define _[_A-Z]'

However, for historical reasons, the preprocessor defines things like "unix"
and "linux" or "AVR" (with avr-gcc). I'm sure other show up on other systems
that I have not tested.

This has been referenced slightly in #2069, but the solution there was to just
manually un-define (-U) the extra define. (Which I'll do if I have to)

This is also talked about briefly in the documentation where it suggest better
alternatives like __unix__ et al. It even says "The C standard requires that
all system-specific macros be part of the reserved namespace."

https://gcc.gnu.org/onlinedocs/cpp/System-specific-Predefined-Macros.html

Could these defines outside of the reserved namespace be removed? Or deprecated
now (if they aren't already?) and removed in 5.0?

Or are those defines here to stay for historical reasons?