changed: use std::filesystem instead of boost::filesystem

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.

opm/common/utility/FileSystem.hpp

@@ -0,0 +1,46 @@
+/*
+  Copyright 2019 Equinor ASA
+
+  This file is part of the Open Porous Media project (OPM).
+
+  OPM is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  OPM is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with OPM.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef OPM_FILESYSTEM_HPP
+#define OPM_FILESYSTEM_HPP
+
+
+#if __cplusplus__ >= 201703L
+#include <filesystem>
+#else
+#include <experimental/filesystem>
+#endif
+
+#include <string>
+
+
+namespace Opm
+{
+#if __cplusplus__ >= 201703L
+    namespace filesystem = std::filesystem;
+#else
+    namespace filesystem = std::experimental::filesystem;
+#endif
+
+    // A poor man's filesystem::unique_path
+    std::string unique_path(const std::string& input);
+
+} // end namespace Opm
+
+
+#endif //  OPM_FILESYSTEM_HPP