Use standard header files to define NULL.

src/device.c

@@ -20,11 +20,10 @@
  */
 
 #include <assert.h>
+#include <stdlib.h>
 
 #include "device-private.h"
 
-#define NULL 0
-
 
 void
 device_init (device_t *device, const device_backend_t *backend)

src/parser.c

@@ -19,9 +19,10 @@
  * MA 02110-1301 USA
  */
 
+#include <stdlib.h>
+
 #include "parser-private.h"
 
-#define NULL 0
 
 void
 parser_init (parser_t *parser, const parser_backend_t *backend)