Add modular library level build file.

[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:

• https://github.com/boostorg/boost/pull/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]

You can fix the clang-7/8 failures by removing 2a from cxxstd (hardly anyone would use clang 7 with -std=c++2a so there's no point wasting time to "properly" fix that config.)

As for msvc-14.1, it's just not present on windows-2019. Probably just remove it and add msvc-14.3 on windows-2022 or 2025 instead.

msvc-14.2 is going to stop working soon too, because windows-2019 is being retired.

[grafikrobot]

@pdimov did that.