Oops, this is supposed to be disabled by default.
diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c
index 090abda..3ee21e4 100644
--- a/Objects/obmalloc.c
+++ b/Objects/obmalloc.c
@@ -541,8 +541,8 @@
 /* This is only useful when running memory debuggers such as
  * Purify or Valgrind.  Uncomment to use.
  *
- */
 #define Py_USING_MEMORY_DEBUGGER
+ */
 
 #ifdef Py_USING_MEMORY_DEBUGGER