[llvm] [third-party] Add a snapshot of Boost.Math 1.89 standalone (PR #141508)

[Louis Dionne via llvm-commits]

ldionne wrote:

> @tstellar Are we going to need an option to build against a system-provided Boost.Math instead of the vendored one? (Though that's not really for this PR but the libcxx integration.)

I think we shouldn't. The way we've envisioned this is that Boost.Math becomes a part of libc++ itself just as if we had copy-pasted the sources. We might need to apply some changes to Boost.Math to integrate it, etc.

It's also possible that libc++ would only work with a specific version of Boost.Math, so then you'd have to match the exact version on your system. All in all, I don't think it's impossible, but it seems to me that making it possible to build against a system-provided Boost goes against the philosophy of the integration we discussed in the RFC.

If that's requested, I'd like to understand the rationale for that need.

https://github.com/llvm/llvm-project/pull/141508