[Bug libstdc++/100146] __cpp_lib_to_chars not defined

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100146

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It's std::from_chars which incorrectly allocates. It uses strtod which requires
a null-terminated string.