stdout log emitter: flush stdout after every printf() if stdout is not a terminal

This makes it possible to watch aegisub's output in realtime even if its
stdout is redirected to a file.

Originally committed to SVN as r6793.

aegisub/libaegisub/unix/log.cpp

@@ -46,6 +46,8 @@ void EmitSTDOUT::log(SinkMessage *sm) {
 		sm->line,
 		(int)sm->len,
 		sm->message);
+	if (!isatty(fileno(stdout)))
+		fflush(stdout);
 }
 
 	} // namespace log