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.

[pdimov]

Are these dependencies actually required? I see nothing in the PR that depends on them.

Conversely, doesn't it depend on the PRs for the library dependencies being merged first? Where does e.g. /boost/mp11 come from? Although CI seems to have no problem with it for some reason.

[grafikrobot]

Are these dependencies actually required? I see nothing in the PR that depends on them.

According to https://pdimov.github.io/boostdep-report/develop/variant2.html yes.

Conversely, doesn't it depend on the PRs for the library dependencies being merged first? Where does e.g. /boost/mp11 come from? Although CI seems to have no problem with it for some reason.

Those get created by the boost-root/jamroot as needed until the transition is fully complete. Although I missed some cases that I'll need to fix. But that was the point of creating all the PRs. To shake out all the issues.

[pdimov]

According to https://pdimov.github.io/boostdep-report/develop/variant2.html yes.

I meant the PR dependencies listed above:

boostorg/boost#854
boostorg/auto_index#7
boostorg/bcp#20
boostorg/boost_install#69
boostorg/boost#890
boostorg/boostbook#25
boostorg/boostdep#21
boostorg/docca#143
boostorg/inspect#19
boostorg/quickbook#25

Why would this PR depend on the tools? It seems to work as-is.

[grafikrobot]

Why would this PR depend on the tools? It seems to work as-is.

It may well not depend on them. But I can't guarantee that building docs will work for any particular library without the tools first. As most libraries do not check if their docs build in CI.

[pdimov]

I don't think that build.jam should affect the docs, regardless of whether tools are updated.

(It currently does, but it shouldn't.)

[grafikrobot]

Please review and merge this PR at your earliest convenience.