[Bug libstdc++/67066] libstdc++-v3/src/filesystem/dir.cc fails to compile with --enable-concept-checks

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67066

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I don't think the --help output is the right place to explain it. My preference
would be to simply deprecate the option anyway.