* Make the library modular usable.
* Switch to library requirements instead of source. As source puts extra source in install targets.
* Add requires-b2 check to top-level build file.
* Bump B2 require to 5.2
* Change all <source> references to <library>.
* Update copyright dates.
* Move inter-lib dependencies to a project variable and into the build targets.
* Replace GHA CI with simpler working one based on alandefreitas/cpp-actions utilities.
* Move project global include to target local include.
* Only msvc gets minor version tests.
* Redo GHA CI to undo move to cpp-actions.
* Add missing deps.
* Add missing deps.
* Explicit g++-* compiler to avoid mismatch errors, and for consistency.
* Fix gcc availability on some OS versions.
* Undo explicit g++ compiler specification.
* Switch to cxxstd.
* Add missing fetch of deps.
