* 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.
* Add missing test deps.
* Bump B2 require to 5.2
* Fix duplicate def of boost.process.fs feature.
* Add missing boost_test dependency.
* Move inter-lib dependencies to a project variable and into the build targets.
* Switch to /boost/test//included target for header only mode of Boost.Test.
* Adjust doc build to avoid boost-root references.
* Update build deps.
* Fix link and build of deps.
