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.

[anarthal]

Looks like the docs build broke: https://drone.cpp.al/boostorg/mysql/784/39/2

[grafikrobot]

Looks like the docs build broke: https://drone.cpp.al/boostorg/mysql/784/39/2

Indeed. I'll be making some changes very soon (which Peter also brought up as a problem) to deal with that.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.03%. Comparing base (00bc812) to head (a7604c4).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #320   +/-   ##
========================================
  Coverage    99.03%   99.03%           
========================================
  Files          144      144           
  Lines         7122     7122           
========================================
  Hits          7053     7053           
  Misses          69       69           

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[grafikrobot]

Please review and merge this PR at your earliest convenience.