Update boost to 1.71 and add asio

boost/math/tools/real_cast.hpp

@@ -0,0 +1,31 @@
+//  Copyright John Maddock 2006.
+//  Use, modification and distribution are subject to 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)
+
+#ifndef BOOST_MATH_TOOLS_REAL_CAST_HPP
+#define BOOST_MATH_TOOLS_REAL_CAST_HPP
+
+#include <boost/math/tools/config.hpp>
+
+#ifdef _MSC_VER
+#pragma once
+#endif
+
+namespace boost{ namespace math
+{
+  namespace tools
+  {
+    template <class To, class T>
+    inline BOOST_MATH_CONSTEXPR To real_cast(T t) BOOST_NOEXCEPT_IF(BOOST_MATH_IS_FLOAT(T) && BOOST_MATH_IS_FLOAT(To))
+    {
+       return static_cast<To>(t);
+    }
+  } // namespace tools
+} // namespace math
+} // namespace boost
+
+#endif // BOOST_MATH_TOOLS_REAL_CAST_HPP
+
+
+