setup.py already does that but only for builds on OS X (darwin); see http://hg.python.org/cpython/file/default/setup.py#l539.  The comments there suggest to me that the code was made conditional to OS X to avoid introducing regressions in builds on other platforms.  The restriction could be removed but probably only for a feature release (e.g. 3.5).  Another approach that has been suggested is to add either configure options (like in Issue21541) or environment variables (Issue5575).  In theory, the best approach might be to use the customization features provided by Modules/Setup*; unfortunately, at the moment there are some major problems with that in current Python 3.4 releases (see Issue17095 for a proposed patch for 3.4).