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.

[grafikrobot]

Please review and merge this PR at your earliest convenience.

[HDembinski]

I didn't see this, because I am not getting notifications when a PR is opened unless someone mentions me with @HDembinski. I changed something in the options of this repo, putting my email address in explicitly under email notifications. Perhaps that is what was missing.

@grafikrobot Thank you for the changes to the build. There seems to be an issue with boost.random in the slow tests, a linker error. Do you have an idea how to fix this? You also remove some extra warning flags cxxflags=-Wno-unused-parameter cxxflags=-Wno-uninitialized, why?

[grafikrobot]

You also remove some extra warning flags cxxflags=-Wno-unused-parameter cxxflags=-Wno-uninitialized, why?

Those are for avoiding warnings in Math (IIRC) that cause errors because of the warnings-as-errors option.