gh-116984: Install mimalloc headers

[oraluben]

This PR installs mimalloc headers and allow extensions that access pycore_*.h continue working.

Such extensions need to explicitly add internal/mimalloc to its header search path.

• Issue: Mimalloc header is not installed #116984

[colesbury]

I think we should only install the headers if Python is built with mimalloc (i.e., not configured with --without-mimalloc).

[colesbury]

This looks good to me, but I'll wait for other people to leave their feedback.

As written in the devguide, for users, the internal API is sometimes "the best workaround for a thorny problem", so we want it to compile successfully in an Python installation.

At the same time, we don't always prefix names with Py or _Py in the internal API, so including CPython internal headers can lead to language level name conflicts. In this case, if an extensions includes our internal headers and bundles mimalloc, they could run into problems. While not ideal, I don't think that's a reason not to make the internal headers compile at all in installations.

[oraluben]

Hi @vstinner , would you like to help landing this?

[erlend-aasland]

Thanks @oraluben and @colesbury!