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.

[Flamefire]

It seems that now the warning settings get propagated to dependencies which cause failures when build libraries that have a looser warning setting than this one.

I.e. I want this library to emit no warnings testing it via -Werror but ignore any warnings from dependencies, especially those in the sources of the dependencies, e.g. now in libs\filesystem\src\operations.cpp, see boostorg/filesystem#321

Can this be done?

The GHA failures should be fixed in develop, no need to worry about those.

[Flamefire]

Any reason for the variable instead of inlining in build/Jamfile?

[grafikrobot]

The only reason is to advertise the dependencies to others consistently for both buildable and header only libs.

[Flamefire]

So that's intended for ("others"=)humans, isn't it? Because otherwise I don't see how this is consistent with e.g. https://github.com/boostorg/variant2/blob/a4b167b7238781d75df5d6684c9e9d40b26456dc/build.jam#L17 given that are different files

My hesitation is because now when reading/editing the build/Jamfile I need to switch to this to see the definition of a variable used there. For head-only libs it is defined and used in the same file (as the build-Jamfile doesn't exist)

To be clear: I'm not saying this is bad, I just want to understand the motivation why.

[grafikrobot]

Yes, it's mainly for people reading the file. I understand the concern though. The attempt is to make this uniform for all libraries (it was easier to generate all of this info this way also).

[Flamefire]

@grafikrobot Anything new about not "up-propagating" warning settings? See https://github.com/boostorg/nowide/actions/runs/10547775762/job/29220866970?pr=183

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.45%. Comparing base (43f53ec) to head (756e3dc).

Additional details and impacted files

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  Coverage    99.45%   99.45%           
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  Files           34       34           
  Lines         3319     3319           
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  Hits          3301     3301           
  Misses          18       18           

[Flamefire]

@grafikrobot See https://github.com/boostorg/nowide/pull/183/files#r1730327788

Why is boost_dependencies a constant and not just inlined in the single place it is used?