What I'd prefer to look for the compiler:

* in distutils: if $CC is an absolute path and exists, use that

* look for clang on $PATH,  use it if found
   (default configure looks for GCC in preference of other compilers,
    but with Apple's toolchain it is better to use clang)

* look for gcc on $PATH, use it if found
   (needed to use the Fink compiler when Fink is on $PATH, alternative
    for 3.3 is to document that configure will try to pick and Apple
    compiler)

* use $(xcodebuild -find clang)

To look for the SDK:

* in distutils: if -isysroot is set, is not '/' and exists, use that

* in distutils: extract SDK version from -isysroot string (if set),
  then use $(xcode-select) to find the root of the installation and
  construct a path relative to that. If the requested SDK version exists,
  use that

* if osx release < 10.5 and building universal: 

  - use /Developer/SDKs/MacOSX10.4u.sdk
    (needed for universal builds on early releases of OSX 10.4)

* if /usr/include/stdint.h exists: use -isysroot /
  (older Xcode, or Xcode >=4.3 with Command Line Tools installed)

* use $(xcodebuild -showsdks) to find list of SDKs and use most recent
  one.


I haven't tried to capture this in code yet, and haven't tested the procedure on earlier releases (or even a 10.7 system with Xcode 4.3 without unix tools), but this should work.