define NULL as nullptr when used in C++11 or later

This should be safer for casting and more compatible with existing code
bases that wrongly assume it must be defined as a pointer.

include/string.h

@@ -7,7 +7,9 @@ extern "C" {
 
 #include <features.h>
 
-#ifdef __cplusplus
+#if __cplusplus >= 201103L
+#define NULL nullptr
+#elif defined(__cplusplus)
 #define NULL 0L
 #else
 #define NULL ((void*)0)