* Make the library modular usable.
* Switch to library requirements instead of source. As source puts extra source in install targets.
* Add missing import-search for cconfig/predef checks.
* Add requires-b2 check to top-level build file.
* Update dependencies.
* Bump B2 require to 5.2
* Remove repeated import-search.
* Move inter-lib dependencies to a project variable and into the build targets.
* Adjust doc build to avoid boost-root references.
* Update build deps.
As suggested by Peter Dimov on boost-dev ML, scope_check functionality
is merged into scope_exit. Thus scope_exit now has an optional condition
function object, which by default always returns true. If a custom
function object is specified, scope_exit works equivalently to the
previous scope_check, i.e. calls the condition function object to check
whether the action function object needs to be called.
