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/sem2.c

@@ -28,7 +28,6 @@
  */
 
 
-#include <stddef.h>
 #include "atom.h"
 #include "atomsem.h"
 #include "atomtests.h"
@@ -321,4 +320,4 @@ static void testCallback (POINTER cb_data)
          */
     }
 
-}
+}