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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.54%. Comparing base (24b61d2) to head (966fcaf).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #218   +/-   ##
========================================
  Coverage    94.54%   94.54%           
========================================
  Files           67       67           
  Lines         8490     8490           
========================================
  Hits          8027     8027           
  Misses         463      463           

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[grafikrobot]

Please review and merge this PR at your earliest convenience.

[mborland]

Seems to be fine. @sdarwin can you please take a look at the Drone run? The x64 Linux builds all fail to clone for out of disk space.

[sdarwin]

@mborland that was 23 days ago. Problem fixed. I have just clicked 'restart' on the job. If you are an admin of the boostorg/charconv repository, you should have the permission in Drone also. "Cancel" or "Restart" are in the top right corner.

[mborland]

@mborland that was 23 days ago. Problem fixed. I have just clicked 'restart' on the job. If you are an admin of the boostorg/charconv repository, you should have the permission in Drone also. "Cancel" or "Restart" are in the top right corner.

Thanks. Since it just turned to reviewable I didn't check the date. My bad.