Add dll_test (to check for visibility issues)

test/dll_test_main.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/core/lightweight_test.hpp>
+
+boost::shared_ptr<int> dll_test();
+
+int main()
+{
+    boost::shared_ptr<int> p1 = dll_test();
+
+    BOOST_TEST_EQ( *p1, 42 );
+
+    return boost::report_errors();
+}