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.

[codecov[bot]]

Codecov Report

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

Project coverage is 87.31%. Comparing base (9735aed) to head (21f7edf). Report is 9 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #102      +/-   ##
===========================================
+ Coverage    85.21%   87.31%   +2.10%     
===========================================
  Files           10       10              
  Lines          879      859      -20     
===========================================
+ Hits           749      750       +1     
+ Misses         130      109      -21     

see 6 files with indirect coverage changes

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[grafikrobot]

Please review and merge this PR at your earliest convenience.