Change to C99 and add weak references

src/allocator.c

@@ -10,8 +10,8 @@
  * Initial value of #refcount_global_allocator.
  */
 static const RefcountAllocator DEFAULT_ALLOCATOR = {
-    .malloc = malloc,
-    .free = free,
+    .malloc.no_data = malloc,
+    .free.no_data = free,
     .pass_data = false,
     .user_data = NULL,
 };