when using clang, whether or not we should link to stdc++fs
cannot easily be detected. it depends on the c++ lib used
(libstdc++ vs libc++), as well as the versions of these.
to avoid messing up in general, this adds an option for forcing
linking to stdc++fs, even when using clang. it defaults to false
to not break existing build setups where it works. is required
for building on jenkins
since we still support g++-7, where filesystem is marked experimental,
we introduce a wrapper header and expose the namespace to use
as Opm::filesystem.
for gcc we unconditionally link with libstdc++fs in the python bindings.
the setup.py stuff links as c code, not c++ code, so it is not
automatically added on any gcc version. this might prove unportable
later.
