--- a/sysdep/unix/io.c
+++ b/sysdep/unix/io.c
@@ -11,6 +11,8 @@
    if _GNU_SOURCE is not defined. */
 #define _GNU_SOURCE 1
 
+#define _GNU_SOURCE 1
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>