Add whatsnew entry

Author: tiran

Doc/whatsnew/3.12.rst

@@ -353,6 +353,12 @@ Changes in the Python API
 Build Changes
 =============
 
+* Python no longer uses ``setup.py`` to build shared C extension modules.
+  Build parameters like headers and libraries are detected in ``configure``
+  script. Extensions are built by ``Makefile``. Most extensions use
+  ``pkg-config`` and fall back to manual detection.
+  (Contributed by Christian Heimes in :gh:`93939`.)
+
 * ``va_start()`` with two parameters, like ``va_start(args, format),``
   is now required to build Python.
   ``va_start()`` is no longer called with a single parameter.