[Bug libstdc++/90612] std::filesystem::path crash

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90612

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. for GCC9 you don't need to use -lstdc++fs for std::filesystem, only for
std::experimental::filesystem.