roken: don't ignore HAVE_OPENPTY on linux

openpty() is not available on all Linux distributions. Trust autoconf's
determination for HAVE_OPENPTY instead of unconditionally using
openpty() on all Linux.

lib/roken/rkpty.c

@@ -99,7 +99,7 @@ open_pty(void)
     printf("implement open_pty\n");
     exit(77);
 #endif
-#if defined(HAVE_OPENPTY) || defined(__linux) || defined(__osf__) /* XXX */
+#if defined(HAVE_OPENPTY) || defined(__osf__) /* XXX */
     if(openpty(&master, &slave, line, 0, 0) == 0)
 	return;
 #endif /* HAVE_OPENPTY .... */