Add void_obj option

include/refcount/refcount.h

@@ -51,6 +51,8 @@ typedef void (*refcount_destroy_callback_t)(void *obj, void *user_data);
  */
 typedef struct RefcountContext {
     size_t entry_offset; //!< Offset to the #RefcountEntry member.
+    void *void_val; //!< Value returned by #refcount_context_unref when a
+                    //!< reference count falls to 0.
     refcount_held_refs_callback_t
         held_refs_callback; //!< Callback to list an object's held references.
     refcount_destroy_callback_t
@@ -75,7 +77,7 @@ typedef struct RefcountContext {
 extern RefcountContext *refcount_default_context;
 
 RefcountContext *
-refcount_make_context(size_t entry_offset,
+refcount_make_context(size_t entry_offset, void *void_val,
                       refcount_held_refs_callback_t held_refs_callback,
                       refcount_destroy_callback_t destroy_callback,
                       void *user_data, const RefcountAllocator *alloc);