https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109818
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And the $build-dir/$target/libstdc++-v3/config.log file should show the output
from this:
AC_MSG_NOTICE("C" header strategy set to $enable_cheaders)
That would confirm if it's being set to c_std
