Update boost to 1.68.0

Keeps the boost libraries up to date. This also silences informational
messages that get spammed throughout the build, such as:

"Info: Boost.Config is older than your compiler version - probably
nothing bad will happen - but you may wish to look for an update Boost
version.  Define BOOST_CONFIG_SUPPRESS_OUTDATED_MESSAGE to suppress this
message."

Which makes the compilation process a lot less noisy on Windows. It's
now much easier to actually spot warnings that occur.

boost/config/header_deprecated.hpp

@@ -0,0 +1,26 @@
+#ifndef BOOST_CONFIG_HEADER_DEPRECATED_HPP_INCLUDED
+#define BOOST_CONFIG_HEADER_DEPRECATED_HPP_INCLUDED
+
+//  Copyright 2017 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
+//
+//  BOOST_HEADER_DEPRECATED("<alternative>")
+//
+//  Expands to the equivalent of
+//    BOOST_PRAGMA_MESSAGE("This header is deprecated. Use <alternative> instead.")
+//
+//  Note that this header is C compatible.
+
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_ALLOW_DEPRECATED_HEADERS)
+# define BOOST_HEADER_DEPRECATED(a)
+#else
+# define BOOST_HEADER_DEPRECATED(a) BOOST_PRAGMA_MESSAGE("This header is deprecated. Use " a " instead.")
+#endif
+
+#endif // BOOST_CONFIG_HEADER_DEPRECATED_HPP_INCLUDED