I'm about to enable support for parallel CPython builds. Thisfunction was currently not safe if called in parallel. This wascausing setuptools / pip downloads to race writing to the same fileand failing builds.

This is less ergonomic (oh Make). But it will make it easier tobuild multiple Python versions at the same time in a future commit.

Previously we generated the same Makefile for every Python version.We can simplify things by only writing 1 Makefile per targetconfiguration.

This refactor allows targets for each major Python version to bedefined simultaneously. This opens the door to a single `make`invocation building multiple versions in parallel.

`build-main.py` now passes in space delimited Python versions tobuild, which the Makefile can happily translate to appropriate maketargets for us.We added support for `--python all` to allow building all supportedPython versions.

Our build system supports this now. So update CI to make use ofthe feature.On Linux, 3.13 freethreaded builds are still their own CI job. Wemay need to tweak `build-main.py` to allow intelligently selectingfreethreaded compatible versions. We could potentially also allowbuilding freethreaded in parallel with non-freethreaded. This can bedone as a followup.