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.

[cppalliance-bot]

An automated preview of the documentation is available at https://60.static-string.prtest.cppalliance.org/libs/static-string/doc/html/index.html

[cppalliance-bot]

An automated preview of the documentation is available at https://60.static-string.prtest.cppalliance.org/libs/static-string/doc/html/index.html

[cppalliance-bot]

An automated preview of the documentation is available at https://60.static-string.prtest.cppalliance.org/libs/static-string/doc/html/index.html

[cppalliance-bot]

An automated preview of the documentation is available at https://60.static-string.prtest.cppalliance.org/libs/static-string/doc/html/index.html

[sdkrystian]

Minor nit, otherwise looks good to me

[cppalliance-bot]

An automated preview of the documentation is available at https://60.static-string.prtest.cppalliance.org/libs/static-string/doc/html/index.html

[grafikrobot]

Please review and merge this PR at your earliest convenience.