Improve support for platforms without stddef.h. NULL definition should now be provided by architecture port file atomport.h, which in most cases can just include stddef.h.

tests/timer5.c

@@ -28,7 +28,6 @@
  */
 
 
-#include <stddef.h>
 #include "atom.h"
 #include "atomsem.h"
 #include "atomtimer.h"
@@ -138,4 +137,4 @@ static void testCallback (POINTER cb_data)
 {
     /* Callback was called */
     callback_ran_flag = TRUE;
-}
+}