Write all messages to stderr.

Messages written to stdout are undesirable when the output of an
application needs to be passed through a pipe.

src/utils.c

@@ -67,7 +67,7 @@ int message (const char* fmt, ...)
 	}
 
 	va_start (ap, fmt);
-	int rc = vfprintf (stdout, fmt, ap);
+	int rc = vfprintf (stderr, fmt, ap);
 	va_end (ap);
 
 	return rc;