[Bug c++/60304] Including <stdbool.h> disables -Wconversion-null in C++ 98 mode

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60304

--- Comment #32 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Nice analysis. Personally I dislike when you get different results from
separate preprocessing, but I don't know if it should be considered a bug.