gh-122584: Import mimalloc headers in a C++ context

[mdboom]

The vendored mimalloc headers are designed to be used in a C++ context if imported from C++. Therefore, we should switch the context back to C++ when using a C++ compiler here. This seems like the most future-proof fix, because otherwise we would need to be careful what context we import pycore_mimalloc.h from.

• Issue: "template with C linkage" error when including "pycore_interp.h" from C++ file #122584

[colesbury]

This seems like an okay workaround , but I think the underlying problems is that we really shouldn't be including headers under an extern "C". I've raised this issue before:

• Move #include directives before extern "C" { in internal headers #115105

It can (and has) caused problems with system headers too.

[vstinner]

Nice workaround.

I tried modifying the 123 header files in Include/internal/ to move #include outside extern "C" { ... }. It's a lot of work, it will be hard to maintain in the future, I don't think that it's worth it.

[cdce8p]

What's the status here? This would also resolve one of the remaining issues with making mypyc compatible with 3.13. Would be great if it could be included in 3.13.0rc2. #121489 (comment)

[miss-islington-app]

Thanks @mdboom for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[bedevere-app]

GH-123035 is a backport of this pull request to the 3.13 branch.