Add a json_c_version.h header (included from json.h), and several macros and functions for retrieving the json-c version at compile-time and run-time.

json.h

@@ -25,6 +25,7 @@ extern "C" {
 #include "json_object.h"
 #include "json_tokener.h"
 #include "json_object_iterator.h"
+#include "json_c_version.h"
 
 #ifdef __cplusplus
 }