find_library does not find stdc++fs

[ben.boeckel (Ben Boeckel (Kitware))]

@JKaniarz

What’s specifically weird is Cray’s Clang 12 requires it when Clang’s documentation says it’s only required in Clang 9

Clang docs are probably talking about libc++. If Cray is using an older libstdc++, even newer Clang would need to link AFAIK (though a link to the docs in question would help).