Is anyone doing and publicizing the git clone vs. tarball work in other projects? I suppose if something has been found, it might be too soon to disclose publicly.
Without that process the options are committing generated files (boo) and burdening those that would build from a git clone instead of a distribution tarball with the demands of details from the development environment. That's intentionally a little vague because of the variety of things that can ultimately affect the autotools output.
All of this is kind of the design of the autotools build process and could very well be a reason to take a harder look at it and the tradeoffs incumbent on its use.
In this particular case xz-utils actually did support CMake for building liblzma, so that's what you use if you don't care about the command line tools.
It's also worth appreciating that in the case of the xz project, the CMake flow is secondary other than for Windows. While I have no evidence one way or another, I wonder if the period in the CMake test that disabled Linux Landlock was actually intentional. Since it disabled it for the CMake build and not the autotools build, it seems like it could have been accidental, unless some significant liblzma consumer builds with CMake and would have otherwise had Landlock enabled.