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]

I don't think the changes from toolset: gcc-x to toolset: gcc; compiler: g++-x are necessary.

[pdimov]

The CI here used both standard and cxxstd with only the former working :-)

Unless @glenfe has an opinion, I suspect it will be less error-prone to switch to cxxstd because that's what everyone else uses and it will inevitably get back here via copy-paste.

[pdimov]

Same here, it's ready, except for Appveyor.