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.

[grafikrobot]

Please review and merge this PR at your earliest convenience.

[pdimov]

This changes CI to use cpp-actions, which has nothing to do with modularity and should be a separate PR (which I'm not going to merge.)

[grafikrobot]

It was the only way I could get the tests to pass. If you are willing to merge this without the tests passing I can undo the CI changes.

[pdimov]

Why the change from toolset: gcc-4.8 to toolset: gcc; compiler: g++-4.8? The former should work.

[grafikrobot]

Why the change from toolset: gcc-4.8 to toolset: gcc; compiler: g++-4.8? The former should work.

IIRC one of the runs like that didn't actually work. So switched to being explicit.

[grafikrobot]

PS. Technically it should not work. As that toolset ends up in user-config. And that should use the plain toolset names to import. The versioned gcc-x.y only works in the CLI feature spec which gets parsed and auto-initialized as given.