Add support for modular build structure.

[grafikrobot]

This is part of the effort to make the Boost libraries "modular" for build and consumption. See https://lists.boost.org/Archives/boost/2024/01/255704.php and https://github.com/grafikrobot/boost-b2-modular/blob/b2-modular/README.adoc for more information.

This PR depends on the following other PRs being merged to both develop and master branches of the respective repos:

• Add support for modular build structure. boost#854

This PR will be changed to ready for review, i.e. not draft, when the above are merged. Do not merge this one until that time.

[pdimov]

What's the problem with libs/functional here?

[grafikrobot]

What's the problem with libs/functional here?

The problem is that this: https://github.com/boostorg/smart_ptr/actions/runs/10429524218/job/28887047180?pr=113#step:7:18 does not install functional. Which is needed for some of the tests.

[pdimov]

Why do you think it's needed?

[grafikrobot]

Why do you think it's needed?

• https://github.com/boostorg/smart_ptr/blob/develop/test/ip_hash_test.cpp#L12
• https://github.com/boostorg/smart_ptr/blob/develop/test/sp_hash_test.cpp#L12
• https://github.com/boostorg/smart_ptr/blob/develop/test/sp_hash_test2.cpp#L12
• https://github.com/boostorg/smart_ptr/blob/develop/test/wp_hash_test.cpp#L7
• https://github.com/boostorg/smart_ptr/blob/develop/test/sp_hash_test3.cpp#L12

[pdimov]

This header isn't in libs/functional.

[grafikrobot]

This header isn't in libs/functional.

Seriously :-( After searching I see it's container_hash. So messy for the brain. I'll fix it. And the other places where this happens.

[grafikrobot]

Please review and merge this PR at your earliest convenience.

[pdimov]

This still contains a lot of unnecessary CI changes.

[grafikrobot]

This still contains a lot of unnecessary CI changes.

Grr.. I thought I had fixed that.

[grafikrobot]

@pdimov okay, removed them. Dealing with >150 PRs has seriously jumbled my brain at this point. :-(

[pdimov]

Good thing I only need to deal with ~23 of these or so.