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.

[jzmaddock]

Hi @grafikrobot and thanks for this, two quick questions:

1. Why the extra dependencies libs/static_assert libs/throw_exception libs/preprocessor when we really shouldn't have any pretty much.

2. I see project requirements clause now has:

project
    : requirements
      <toolset>gcc:<cxxflags>-Wno-deprecated-declarations
      <library>/boost/core//boost_core
      <library>/boost/detail//boost_detail
      <library>/boost/type_traits//boost_type_traits
;

Again why the need?

I note that a similar clause in Boost.Math just recently completely broken our CI, so I removed it: boostorg/math#1254. I wondered where that had come from ;) Was this wrong?? The problem is it's a heavyweight dependency that makes every target in the Jamfile depend on the properties of those other libraries, rather than just the targets that actually need it.

Thanks!

[grafikrobot]

1. Why the extra dependencies libs/static_assert libs/throw_exception libs/preprocessor when we really shouldn't have any pretty much.

Because Boost.Core needs them. And the CI runs fail if they don't have those.

[jzmaddock]

Merged from parallel PR.