Implement hardware_concurrency on Windows.

asio/include/asio/detail/impl/win_thread.ipp

diff --git a/asio/include/asio/detail/impl/win_thread.ipp b/asio/include/asio/detail/impl/win_thread.ipp
index 554f9f6..6996f40 100644
--- a/asio/include/asio/detail/impl/win_thread.ipp
+++ b/asio/include/asio/detail/impl/win_thread.ipp
@@ -53,6 +53,13 @@
   }
 }
 
+std::size_t win_thread::hardware_concurrency()
+{
+  SYSTEM_INFO system_info;
+  ::GetSystemInfo(&system_info);
+  return system_info.dwNumberOfProcessors;
+}
+
 void win_thread::start_thread(func_base* arg, unsigned int stack_size)
 {
   ::HANDLE entry_event = 0;

asio/include/asio/detail/win_thread.hpp

diff --git a/asio/include/asio/detail/win_thread.hpp b/asio/include/asio/detail/win_thread.hpp
index 0f8abc4..38f60a7 100644
--- a/asio/include/asio/detail/win_thread.hpp
+++ b/asio/include/asio/detail/win_thread.hpp
@@ -19,6 +19,7 @@
 
 #if defined(ASIO_WINDOWS) && !defined(UNDER_CE)
 
+#include <cstddef>
 #include "asio/detail/noncopyable.hpp"
 #include "asio/detail/socket_types.hpp"
 
@@ -76,6 +77,9 @@
   // Wait for the thread to exit.
   ASIO_DECL void join();
 
+  // Get number of CPUs.
+  ASIO_DECL static std::size_t hardware_concurrency();
+
 private:
   friend ASIO_DECL unsigned int __stdcall win_thread_function(void* arg);