Issue #13590: Improve support for OS X Xcode 4:

Issue #13590: Improve support for OS X Xcode 4:
- Try to avoid building Python or extension modules with problematic
  llvm-gcc compiler.
- Since Xcode 4 removes ppc support, extension module builds now
  check for ppc compiler support and automatically remove ppc and
  ppc64 archs when not available.
- Since Xcode 4 no longer install SDKs in default locations,
  extension module builds now revert to using installed headers
  and libs if the SDK used to build the interpreter is not
  available.
- Update ./configure to use better defaults for universal builds;
  in particular, --enable-universalsdk=yes uses the Xcode default
  SDK and --with-universal-archs now defaults to "intel" if ppc
  not available.