Great!  http://docs.python.org/devguide contains all the info needed to get the code and make a patch.  If you apply your suggestion to the code and it fixes your build, I will commit it.  A small unit test to check the new behavior of build_ext and avoid regressions will be needed, but it’s not always easy to write these tests, so depending on your time I will be able to provide guidance or write the test myself.