Separate a definition of NULL to standalone header

A lib/list.h had circular dependency through include nest/bird.h
  nest/bird.h requires lib/birdlib.h
  lib/birdlib.h requires timer.h
  timer.h requires lib/resource.h
  lib/resource.h requires lib/list.h

lib/Modules

@@ -25,3 +25,4 @@ event.h
 checksum.c
 checksum.h
 alloca.h
+null.h

lib/birdlib.h

@@ -11,6 +11,7 @@
 
 #include "timer.h"
 #include "alloca.h"
+#include "lib/null.h"
 
 /* Ugly structure offset handling macros */
 
@@ -43,10 +44,6 @@
 #define BIT32_CLR(b,p)		((b)[(p)/32] &= ~BIT32_VAL(p))
 #define BIT32_ZERO(b,l)		memset((b), 0, (l)/8)
 
-#ifndef NULL
-#define NULL ((void *) 0)
-#endif
-
 #ifndef IPV6
 #define IP_VERSION 4
 #else

lib/lists.c

@@ -26,7 +26,7 @@
 
 #define _BIRD_LISTS_C_
 
-#include "nest/bird.h"
+#include "lib/null.h"
 #include "lib/lists.h"
 
 /**

lib/null.h

@@ -0,0 +1,9 @@
+/*
+ *	BIRD Library -- NULL
+ *
+ *	Can be freely distributed and used under the terms of the GNU GPL.
+ */
+
+#ifndef NULL
+#define NULL ((void *) 0)
+#endif