1.56 rc1 ASIO doesn't compile with mingw32

[Niall Douglas]

This is from the most recent mingw, the one with GCC 4.8. Yes I agree mingw is broken :(

In file included from ./boost/asio/detail/win_object_handle_service.hpp:180:0,
                 from ./boost/asio/windows/object_handle_service.hpp:25,
                 from ./boost/asio/windows/basic_object_handle.hpp:27,
                 from ./boost/asio.hpp:108,
                 from ./boost/afio/afio.hpp:24,
                 from libs\afio\test\test_functions.hpp:32:
./boost/asio/detail/impl/win_object_handle_service.ipp: In member function 'void boost::asio::detail::win_object_handle_service::move_construct(boost::asio::detail::win_object_handle_service::implementation_type&, boost::asio::detail::win_object_handle_service::implementation_type&)':
./boost/asio/detail/impl/win_object_handle_service.ipp:106:5: error: '::UnregisterWaitEx' has not been declared
     ::UnregisterWaitEx(impl.wait_handle_, INVALID_HANDLE_VALUE);
     ^
./boost/asio/detail/impl/win_object_handle_service.ipp: In member function 'void boost::asio::detail::win_object_handle_service::move_assign(boost::asio::detail::win_object_handle_service::implementation_type&, boost::asio::detail::win_object_handle_service&, boost::asio::detail::win_object_handle_service::implementation_type&)':
./boost/asio/detail/impl/win_object_handle_service.ipp:158:5: error: '::UnregisterWaitEx' has not been declared
     ::UnregisterWaitEx(impl.wait_handle_, INVALID_HANDLE_VALUE);
     ^
./boost/asio/detail/impl/win_object_handle_service.ipp: In member function 'void boost::asio::detail::win_object_handle_service::destroy(boost::asio::detail::win_object_handle_service::implementation_type&)':
./boost/asio/detail/impl/win_object_handle_service.ipp:200:7: error: '::UnregisterWaitEx' has not been declared
       ::UnregisterWaitEx(wait_handle, INVALID_HANDLE_VALUE);
       ^
./boost/asio/detail/impl/win_object_handle_service.ipp: In member function 'boost::system::error_code boost::asio::detail::win_object_handle_service::close(boost::asio::detail::win_object_handle_service::implementation_type&, boost::system::error_code&)':
./boost/asio/detail/impl/win_object_handle_service.ipp:251:7: error: '::UnregisterWaitEx' has not been declared
       ::UnregisterWaitEx(wait_handle, INVALID_HANDLE_VALUE);
       ^
./boost/asio/detail/impl/win_object_handle_service.ipp: In member function 'boost::system::error_code boost::asio::detail::win_object_handle_service::cancel(boost::asio::detail::win_object_handle_service::implementation_type&, boost::system::error_code&)':
./boost/asio/detail/impl/win_object_handle_service.ipp:302:7: error: '::UnregisterWaitEx' has not been declared
       ::UnregisterWaitEx(wait_handle, INVALID_HANDLE_VALUE);
       ^
./boost/asio/detail/impl/win_object_handle_service.ipp: In static member function 'static void boost::asio::detail::win_object_handle_service::wait_callback(PVOID, BOOLEAN)':
./boost/asio/detail/impl/win_object_handle_service.ipp:402:5: error: '::UnregisterWaitEx' has not been declared
     ::UnregisterWaitEx(impl->wait_handle_, NULL);
     ^