Fix unused return value warning.

asio/include/asio/detail/impl/signal_set_service.ipp

diff --git a/asio/include/asio/detail/impl/signal_set_service.ipp b/asio/include/asio/detail/impl/signal_set_service.ipp
index 4cde184..14adbab 100644
--- a/asio/include/asio/detail/impl/signal_set_service.ipp
+++ b/asio/include/asio/detail/impl/signal_set_service.ipp
@@ -63,8 +63,9 @@
 #else // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
   int saved_errno = errno;
   signal_state* state = get_signal_state();
-  (void)::write(state->write_descriptor_,
+  int result = ::write(state->write_descriptor_,
       &signal_number, sizeof(signal_number));
+  (void)result;
   errno = saved_errno;
 #endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)