This replaces the cxx11_hdr_type_traits requirement with a local
configure-time check for the presence of only those type traits
which are used in the library. This enables testing of gcc 4.8 and
4.9, which was previously silently skipped.
* 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.