Improved support for IBM operating systems

Fix compiler errors and warnings when building on IBM operating systems such as AIX and IBM i.

json_visit.h

@@ -8,6 +8,10 @@
  */
 #include "json_object.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef int(json_c_visit_userfunc)(json_object *jso, int flags, json_object *parent_jso,
                                    const char *jso_key, size_t *jso_index, void *userarg);
 
@@ -90,4 +94,8 @@ JSON_EXPORT int json_c_visit(json_object *jso, int future_flags, json_c_visit_us
  */
 #define JSON_C_VISIT_RETURN_ERROR -1
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* _json_c_json_visit_h_ */