[Bug libstdc++/78870] Support std::filesystem on Windows

[jhasse at bixense dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78870

--- Comment #2 from Jan Niklas Hasse <jhasse at bixense dot com> ---
I'm willing to help. Is there an existing starting point for Windows? Can
boost::filesystem's implementation be used?