[Bug bootstrap/78251] config/gettext.m4 and config/iconv.m4 contaminate CPPFLAGS

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78251

--- Comment #8 from Eric Gallager <egallager at gcc dot gnu.org> ---
r265896 might have affected this