[Bug libstdc++/86756] Don't define __cpp_lib_filesystem unless --enable-libstdcxx-filesystem-ts

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86756

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Thinking further about this, I think <filesystem> should always be installed,
but it should only define the portable parts.

The enumeration types, filesystem::path, filesystem::filesystem_error and
filesystem::directory_entry are all supportable for any hosted implementation.

Directory iterators and filesystem operations require OS support, so should be
conditionally-supported.

The C++17 implementation needs to move from libstdc++fs.a into libstdc++.so
anyway, so that simplifies things a little.

(In reply to Jonathan Wakely from comment #1)
> Same for <experimental/filesystem> and __cpp_lib_experimental_filesystem.

It looks like that is already dependent on ENABLE_FILESYSTEM_TS so nothing to
do there.