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_main.h

@@ -0,0 +1,32 @@
+# /* 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_MAIN_H
+# define BOOST_LIBS_PREPROCESSOR_REGRESSION_TEST_MAIN_H
+#
+#if defined(__cplusplus)
+#include <cstdio>
+#if !defined(_STLP_MSVC) || _STLP_MSVC >= 1300
+namespace std { }
+using namespace std;
+#endif
+#else
+#include <stdio.h>
+#endif
+
+int main(void) {
+    printf("pass " __TIME__);
+    return 0;
+}
+
+# endif