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:

• https://github.com/boostorg/boost/pull/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.

[Flamefire]

@grafikrobot I rebased your branch to remove the sync commits without any changes.

I then fixed the dynamic build as <link>shared:<define>BOOST_LOCALE_DYN_LINK=1 was missing from the usage-requirements, see https://github.com/boostorg/locale/pull/234/commits/3ff8e5a3f029b125f5d286957f979fc71f264cde

Can you verify the current state please?

Is it possible to have keyword arguments for lib in B2 and add the common-requirements there too which currently only exist for project? That would reduce the duplication and avoid the above issue. It also removes the current practice of commenting what the arguments to lib mean.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.76%. Comparing base (84d91b3) to head (3ff8e5a). Report is 11 commits behind head on develop.

Additional details and impacted files

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