So ultimately this is a major sysconfig design flaw, and it likely requires a rewrite.

The mix of information sources between _sysconfigdata (even when overriding the name and PYTHONPATH) and the Makefile, and the order in which you can rely them to be loaded (you can't) and the fact that INSTALL_SCHEMES depends on _neither_ (it only uses the running interpreter's sys module) means you can't possibly get a coherent view of the settings for anything other than the running interpreter.

Since we need something else for cross-compiling, we should either fix sysconfig so that it's possible to override the source of information reliably, or we avoid using it completely for building the CPython extension modules. The former would likely help third-party cross-compilation more.

I'm not sure who needs to be in on this, so I'll post to python-dev before going any further.