[Bug c/124830] c/c-decl.cc Fails to cross compile with C++20 host compiler

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124830

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Can also be reproduced with --disable-bootstrap instead of building a cross
compiler.  When bootstrapping we force -std=c++14 during stage1 build.