fix build with gcc7

the filesystem ts did not include proximate. implement
a poor man's version (copied from libstdc++, slightly modified)
and use this when necessary

tests/test_FileSystem.cpp

@@ -0,0 +1,44 @@
+/*
+   +   Copyright 2021 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/>.
+   +   */
+
+#include "config.h"
+
+#include <opm/common/utility/FileSystem.hpp>
+
+#define BOOST_TEST_MODULE FileSystem
+#include <boost/test/unit_test.hpp>
+
+
+BOOST_AUTO_TEST_CASE(TestProximate) {
+
+    Opm::filesystem::path rel1("/a/b");
+    Opm::filesystem::path input1("/a/b/c");
+    auto test1 = Opm::proximate(input1, rel1);
+    BOOST_CHECK_EQUAL(test1.string(), "c");
+
+    Opm::filesystem::path input2("/a/c");
+    auto test2 = Opm::proximate(input2, rel1);
+    BOOST_CHECK_EQUAL(test2.string(), "../c");
+
+    Opm::filesystem::path input3("c");
+    auto test3 = Opm::proximate(input3, rel1);
+    BOOST_CHECK_EQUAL(test3.string(), "c");
+
+    auto test4 = Opm::proximate(rel1, input3);
+    BOOST_CHECK_EQUAL(test4.string(), "/a/b");
+}