Old gcc versions cannot find std::filesystem

[Alexey-N-Chernyshov]

Issue was opened in Fuhon
gcc prior 8.1 does not support std::filesystem. See C++17 library features
gcc 8.1 - 9.0 requires linking option See notes
gcc 9.0+ has full support, no problems

The best option is to check for std::filesystem support in cmake and link library if required.
At least requirements should be added to README.
Link may be helpful