Release build and debug build are now ABI compatible: the Py_DEBUG
define no longer implies Py_TRACE_REFS define which introduces the
only ABI incompatibility.

A new "./configure --with-trace-refs" build option is now required to
get Py_TRACE_REFS define which adds sys.getobjects() function and
PYTHONDUMPREFS environment variable.

Changes:

* Add ./configure --with-trace-refs
* Py_DEBUG no longer implies Py_TRACE_REFS