Add dll_test (to check for visibility issues)

test/dll_test_lib.cpp

@@ -0,0 +1,20 @@
+// Copyright 2018 Peter Dimov
+//
+// Distributed under the Boost Software License, Version 1.0.
+//
+// See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/shared_ptr.hpp>
+#include <boost/config.hpp>
+
+#if defined(DLL_TEST_DYN_LINK)
+# define EXPORT BOOST_SYMBOL_EXPORT
+#else
+# define EXPORT
+#endif
+
+EXPORT boost::shared_ptr<int> dll_test()
+{
+    return boost::shared_ptr<int>( new int( 42 ) );
+}