The attached patch (issue1390-version1.txt) is not tested very well at the moment because I'm running into another problem when building on OSX 10.5, but should improve the build situation

With this patch:

* Configure will automatically pick clang when gcc is llvm-gcc
  (which miscompiles python3.3)
* Configure will use the compiler embedded in Xcode.app when there is
  no compiler in /usr/bin (e.g. when using a clean install of Xcode 4.3)
* Configure picks the most recent available SDK when using
  --with-universalsdk (cannot use '/' as the -isysroot because 
  Xcode4.3 won't install headers in /usr/include unless you install
  the unix tools)
* Distutils uses the configured compiler when available, and uses the
  same algorithm as configure when the configured compiler cannot be
  found
* Distutils will remove '-arch ppc' from the compiler flags when that
  architecture is not supported (such as with recent Xcode 4 releases)

I have build the tree on 10.7 and 10.6 with working tests. I'm now trying to build on 10.5 to generate the 32-bit installer, but ran into problems on my 10.5 machine: python.exe claims it cannot find the std. library. This is likely the known issue of Apple's GCC miscompiling python, but I haven't excluded other possibilities yet.