[Bug libstdc++/128429] libstdc++ headers not considered system headers in the build tree

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128429

--- Comment #1 from Drea Pinski <pinskia at gcc dot gnu.org> ---
I think this is correct. We don't want to consider them system headers while
building GCC.