Add definition _GNU_SOURCE to remove warnings to do with implicit functions

Signed-off-by: Deokgyu Yang <secugyu@gmail.com>
Change-Id: I02489bde9498c3acb22af381bd4852e5a667c677
diff --git a/wiringPi/wiringPi.c b/wiringPi/wiringPi.c
index 793735e..4c7f65a 100644
--- a/wiringPi/wiringPi.c
+++ b/wiringPi/wiringPi.c
@@ -5,6 +5,10 @@
 
  */
 /*----------------------------------------------------------------------------*/
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
 #include <stdio.h>
 #include <stdarg.h>
 #include <stdint.h>