Thanks for the review of our PR. I think not all points made there are relevant:

* on older systems, where `xcrun --show-sdk-path` is not available, one can assume that /usr/include is there, and simply set the corresponding variable to this value in ./configure

* building Python with a non-system compiler is only possible with system headers, and thus either SDK or at least command line tools, available, and all is well.

* mid-flight switching to a different SDK/compiler is dodgy, and in particular switching to a different set of headers is asking for trouble. And the gcc toolchain is not much different in this way - e.g. on Gentoo one can do gcc-config to switch from one profile to another, with provably "interesting" results if it is done mid-flight.  

* the Python 2.7 zlib issue is not a problem with our PR. 

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Most probably, the desired switch to autotools can only happen gradually, by moving more of setup.py to ./configure, and our PR is a step in such direction.