--- /a/src/libthread/threadimpl.h	2014-03-05 21:43:20.000000000 +0000
+++ /b/src/libthread/threadimpl.h	2017-06-29 03:37:52.410011050 +0000
@@ -1,3 +1,4 @@
+#define _GNU_SOURCE
 #include "u.h"
 #include <errno.h>
 #include <sys/time.h>