OpenBSD is missing SI_ASYNCIO and SI_MESGQ

Check if macros SI_ASYNCIO and SI_MESGQ are defined as macros.  This
allows to run tests on OpenBSD 6.5, where the macros are missing.

This is identical to patch-boost_test_impl_execution_monitor_ipp in
OpenBSD Ports, except that I added a comment.

include/boost/test/impl/execution_monitor.ipp

@@ -391,14 +391,19 @@ system_signal_exception::report() const
         report_error( execution_exception::system_error,
                       "signal: the expiration of a timer set by timer_settimer()" );
         break;
+// OpenBSD was missing SI_ASYNCIO and SI_MESGQ
+#ifdef SI_ASYNCIO
     case SI_ASYNCIO:
         report_error( execution_exception::system_error,
                       "signal: generated by the completion of an asynchronous I/O request" );
         break;
+#endif
+#ifdef SI_MESGQ
     case SI_MESGQ:
         report_error( execution_exception::system_error,
                       "signal: generated by the the arrival of a message on an empty message queue" );
         break;
+#endif
     default:
         break;
     }