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.

[pdimov]

Too much stuff commented out in CI; while I understand why it's done, it's not for me to merge such changes.

[grafikrobot]

Okay then.. @glenfe please merge this PR.

[pdimov]

Way too many unnecessary CI changes... what's the point of renaming cxxstd to standard, for instance? This just generates diffs that have nothing to do with the point of the PR.

[grafikrobot]

Way too many unnecessary CI changes... what's the point of renaming cxxstd to standard, for instance?

As usual.. I just wanted things to work hence copy pasted from something else.

This just generates diffs that have nothing to do with the point of the PR.

LOL I've been doing CI changes only for months. The point of the PR was done long, long, ago. ;-)

[pdimov]

I merged this before noticing that Appveyor needs a fix too.

[grafikrobot]

I merged this before noticing that Appveyor needs a fix too.

I'll make another PR to try and fix Appveyor.