Added a BOOST_DEPRECATED macro for deprecated symbol markup.

BOOST_DEPRECATED can be used to mark functions, types and objects as
deprecated, with a message with a recommendation of replacement. Using
such marked symbols in code will generate compiler warnings, with the
specified message, if possible.

The warnings can be suppressed if BOOST_ALLOW_DEPRECATED_SYMBOLS is defined.
Additionally, added support BOOST_ALLOW_DEPRECATED macro that not only
allows for deprecated symbols but also deprecated headers (i.e. defining
BOOST_ALLOW_DEPRECATED is equivalent to defining both
BOOST_ALLOW_DEPRECATED_SYMBOLS and BOOST_ALLOW_DEPRECATED_HEADERS).

test/symbol_deprecated_test.cpp

@@ -0,0 +1,19 @@
+//  Copyright 2022 Andrey Semashev.
+//
+//  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/config.hpp>
+
+BOOST_DEPRECATED("Use bar() instead.")
+void foo();
+
+template< typename T >
+class BOOST_DEPRECATED("Use std::unique_ptr instead.") my_auto_ptr
+{
+};
+
+BOOST_DEPRECATED("Use std::numeric_limits<int>::max() instead.")
+const int max_int = 0x7fffffff;