[Bug preprocessor/117124] Header file contents get modified when macro defined before header file inclusion

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117124

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Keywords are not reserved identifiers, so for some reason C doesn't say this is
undefined.

I suppose you just get what you deserve when you replace 'extern' with
something that makes the program ill-formed.