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
• https://github.com/boostorg/auto_index/pull/7
• https://github.com/boostorg/bcp/pull/20
• https://github.com/boostorg/boost_install/pull/69
• https://github.com/boostorg/boost/pull/890
• https://github.com/boostorg/boostbook/pull/25
• https://github.com/boostorg/boostdep/pull/21
• https://github.com/boostorg/docca/pull/143
• https://github.com/boostorg/inspect/pull/19
• https://github.com/boostorg/quickbook/pull/25

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.