[Bug libstdc++/86756] New: 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

            Bug ID: 86756
           Summary: Don't define __cpp_lib_filesystem unless
                    --enable-libstdcxx-filesystem-ts
           Product: gcc
           Version: 8.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Currently the <filesystem> header is installed unconditionally, even if the
libstdc++fs.a library is not installed.

Either the headers should not be installed, or the feature test macro should
not be defined.