Fix unused variable warnings.

asio/include/asio/detail/win_iocp_handle_service.hpp

diff --git a/asio/include/asio/detail/win_iocp_handle_service.hpp b/asio/include/asio/detail/win_iocp_handle_service.hpp
index 28b9c2b..bfc159c 100644
--- a/asio/include/asio/detail/win_iocp_handle_service.hpp
+++ b/asio/include/asio/detail/win_iocp_handle_service.hpp
@@ -697,6 +697,8 @@
       impl.safe_cancellation_thread_id_ = ::GetCurrentThreadId();
     else if (impl.safe_cancellation_thread_id_ != ::GetCurrentThreadId())
       impl.safe_cancellation_thread_id_ = ~DWORD(0);
+#else // defined(ASIO_ENABLE_CANCELIO)
+    (void)impl;
 #endif // defined(ASIO_ENABLE_CANCELIO)
   }
 

asio/include/asio/detail/win_iocp_socket_service.hpp

diff --git a/asio/include/asio/detail/win_iocp_socket_service.hpp b/asio/include/asio/detail/win_iocp_socket_service.hpp
index b032ae2..cb1d203 100644
--- a/asio/include/asio/detail/win_iocp_socket_service.hpp
+++ b/asio/include/asio/detail/win_iocp_socket_service.hpp
@@ -1933,6 +1933,8 @@
       impl.safe_cancellation_thread_id_ = ::GetCurrentThreadId();
     else if (impl.safe_cancellation_thread_id_ != ::GetCurrentThreadId())
       impl.safe_cancellation_thread_id_ = ~DWORD(0);
+#else // defined(ASIO_ENABLE_CANCELIO)
+    (void)impl;
 #endif // defined(ASIO_ENABLE_CANCELIO)
   }