fix warning:
libc_wrapper.c:54: warning: implicit declaration of function usleep

--- a/src/backend/libc_wrapper.c
+++ b/src/backend/libc_wrapper.c
@@ -27,6 +27,8 @@
 
+#define _BSD_SOURCE
 #include <stdio.h>
 #include "xfree.h"
+#include "config.h"
 
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>