PR libstdc++/85818 ensure path::preferred_separator is defined

Because path.cc is compiled with -std=gnu++17 the static constexpr
data member is implicitly 'inline' and so no definition gets emitted
unless it gets used in that translation unit. Other translation units
built as C++11 or C++14 still require a namespace-scope definition of
the variable, so mark the definition as used.

	PR libstdc++/85818
	* src/filesystem/path.cc (path::preferred_separator): Add used
	attribute.
	* testsuite/experimental/filesystem/path/preferred_separator.cc: New.

From-SVN: r260326

libstdc++-v3/ChangeLog

 2018-05-17  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/85818
+	* src/filesystem/path.cc (path::preferred_separator): Add used
+	attribute.
+	* testsuite/experimental/filesystem/path/preferred_separator.cc: New.
+
 	PR libstdc++/85812
 	* libsupc++/cxxabi_init_exception.h (__cxa_free_exception): Declare.
 	* libsupc++/exception_ptr.h (make_exception_ptr) [__cpp_exceptions]:

libstdc++-v3/src/filesystem/path.cc

@@ -33,7 +33,7 @@ using fs::path;
 
 fs::filesystem_error::~filesystem_error() = default;
 
-constexpr path::value_type path::preferred_separator;
+constexpr path::value_type path::preferred_separator [[gnu::used]];
 
 path&
 path::remove_filename()

libstdc++-v3/testsuite/experimental/filesystem/path/preferred_separator.cc

+// Copyright (C) 2018 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library 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, or (at your option)
+// any later version.
+
+// This library 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 this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-O0 -lstdc++fs -std=gnu++11" }
+// { dg-do link { target c++11 } }
+
+#include <experimental/filesystem>
+
+std::experimental::filesystem::path::value_type
+test01()
+{
+  auto* sep = &std::experimental::filesystem::path::preferred_separator;
+  return *sep;
+}
+
+int
+main()
+{
+  test01();
+}