The Build Applet.app tool for OS X depends on the deprecated EasyDialogs module to interact with the user.  EasyDialogs depends on Apple-deprecated Carbon QuickDraw APIs.  As of OS X 10.8, the headers for the QuickDraw APIs are no longer supplied with Xcode 4.  This means that Build Applet.app can no longer be built on 10.8 unless an earlier version of Xcode and an SDK from an earlier system are used.  The Mac/Makefile target "install_BuildApplet" fails with either:
  ImportError: cannot import name GetNewDialog
or
  AttributeError: 'module' object has no attribute 'GetQDGlobalsScreenBits'
depending on whether the interpreter being built supports 32-bit or not.    

Given that Build Applet.app is already considered deprecated and has been removed in Python 3, it does not seem appropriate to attempt to re-engineer its GUI interface.  Instead, the Makefile should just skip building it if EasyDialogs is not available.

(Note that the QuickDraw libraries are still being shipped in OS X 10.8, so a "Build Applet.app" built on an older system - such as with the python.org OS X installers - will still run on 10.8.  There is no guarantee that will still be true in future versions of OS X.)