Added OPT macro to determine whether C++20's __VA_OPT__ is supported or not. Updated the undocumented variadic IS_EMPTY to use the functionality of __VA_OPT__ if it exists to provide perfect functionality for testing for emptiness.

test/test_macro.h

@@ -0,0 +1,22 @@
+# /* Copyright (C) 2001
+#  * Housemarque Oy
+#  * http://www.housemarque.com
+#  *
+#  * 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)
+#  */
+#
+# /* Revised by Paul Mensonides (2002) */
+#
+# /* See http://www.boost.org for most recent version. */
+#
+# ifndef BOOST_LIBS_PREPROCESSOR_REGRESSION_TEST_MACRO_H
+# define BOOST_LIBS_PREPROCESSOR_REGRESSION_TEST_MACRO_H
+#
+# include <boost/preprocessor/cat.hpp>
+#
+# define BEGIN typedef int BOOST_PP_CAT(test_, __LINE__)[((
+# define END )==1) ? 1 : -1];
+#
+# endif