updated boost on windows

winx64/include/boost/asio/detail/std_thread.hpp

@@ -2,7 +2,7 @@
 // detail/std_thread.hpp
 // ~~~~~~~~~~~~~~~~~~~~~
 //
-// Copyright (c) 2003-2017 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+// Copyright (c) 2003-2019 Christopher M. Kohlhoff (chris at kohlhoff dot 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)
@@ -52,6 +52,12 @@ public:
       thread_.join();
   }
 
+  // Get number of CPUs.
+  static std::size_t hardware_concurrency()
+  {
+    return std::thread::hardware_concurrency();
+  }
+
 private:
   std::thread thread_;
 };