[libcxx-commits] [libcxx] [libc++] Reduce the dependency of the locale base API on the base system from the headers (PR #117764)

[Louis Dionne via libcxx-commits]

ldionne wrote:

@nico Looking at the CI failures ([on AIX](https://buildkite.com/llvm-project/libcxx-ci/builds/39015#019369ed-4af4-4cb6-9ad0-1f8af62704e8), [on FreeBSD](https://buildkite.com/llvm-project/libcxx-ci/builds/39015#019369ed-4af7-4a31-a38c-100b611cf787) and [on macOS](https://github.com/llvm/llvm-project/actions/runs/12037529175/job/33562360918?pr=117764)), it looks like much of the library was already broken when defining `_XOPEN_SOURCE` to older values. In particular, `<fstream>` makes use of `fseeko` (and I think it has been for a very long time).

I didn't think this was going to be my conclusion, but I am tempted to conclude that such legacy values for this macro might not be worth supporting. Doing it properly certainly seems to require a non-trivial amount of refactoring. Do you have thoughts?

https://github.com/llvm/llvm-project/pull/117764