Add support for modular build structure.

[grafikrobot]

This adds the minimal set of declarations to make it possible to have both legacy non-modular building work with new modular building. It allows incremental addition in libraries to get them ready for modular building.

This is a temporary state though. After all libraries, tools, and testing build in this mixed mode another version of the jamroot will be needed to remove the legacy build handling.

[pdimov]

@mclow , @glenfe , any objections to this?

[grafikrobot]

Preparing a different PR that allows for an easier to manage transition to replace this one.

[grafikrobot]

Close & open for CI to reset.

[mclow]

I've looked at this, and I don't think I know enough about Jamfiles to make reasonable comments.

[grafikrobot]

Okay, this is truly now ready. I've now made all the changes to support both not-modular, fully-modular, and everything in between. You can see the testing in these two repos:

• https://github.com/grafikrobot/boostorg.boost/actions - Has this change and all the libs and tools modified with the regular boost-root.
• https://github.com/grafikrobot/boost-b2-modular/actions/workflows/check-libs.yml - Has a root-less structure with all the libraries, and the boost_install tool as the above. But only with a project-config.jam as a fully-modular setup. Note the CI on this is not expected to pass. As it's trying to fully build everything blindly. And many libs have failing tests in the regular course.

[pdimov]

The change of BOOST_VERSION from 1.85.0 to 1.86.0 is probably no longer necessary.

[grafikrobot]

The change of BOOST_VERSION from 1.85.0 to 1.86.0 is probably no longer necessary.

You could press the update/rebase button to resolve that. I don't get that button apparently.

[grafikrobot]

The change of BOOST_VERSION from 1.85.0 to 1.86.0 is probably no longer necessary.

You could press the update/rebase button to resolve that. I don't get that button apparently.

@pdimov manually merged current develop to resolve that.