* Add modular library level build file.
* Make the library modular usable.
* Add missing import-search for cconfig/predef checks.
* Add requires-b2 check to top-level build file.
* Remove extra import-search.
* Remove extra require-b2.
* Bump B2 require to 5.2
* Update copyright dates.
* Move inter-lib dependencies to a project variable and into the build targets.
* Adjust doc build to avoid boost-root references.
* Update build deps.
* Replace GHA CI with simpler working one based on alandefreitas/cpp-actions utilities.
* GCC 4.8 is the minimum available.
* Move project global include to target local include.
* Only msvc gets minor version tests.
* Redo GHA CI to undo move to cpp-actions.
* Update os/containers to account for unavailable packages in old containers.
* Tweaks per comments.
