Make LLVM_VERSION configurable with env variable

[danigm]

Right now the LLVM version used is hardcoded in _llvm.py file.

The version could be override using environment variables so downstream builders can choose a different version to build against when enabling experimental JIT.

Linked PRs

• gh-138497: Support LLVM_VERSION configuration via env #138498

[sobolevn]

I think that this can lead to a problem that people will supply versions of LLVM that are not supported.

[Fidget-Spinner]

Yeah we don't guarantee that we can build with a range of LLVM versions, only a specific one for the JIT.

[brandtbucher]

I'm honestly on the fence about this. I agree that we should only officially support one version at a time, but in practice a few downstream distributors seem to be patching the LLVM version (either using a newer version on an older Python or an older version on a newer Python).

Allowing the officially supported version to be overridden with an env var is probably okay, provided it's clear that we don't support any version other than the default. Like CFLAGS_JIT, this should probably be captured at configure-time and passed to the build script rather than discovered by the build script itself.