* 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.
Scope guard factory functions used to generate scope guards with incorrect
action and condition types if the factory was called with function
references. The scope guard template arguments used to be function types
instead of references to functions.
Also, deduction guides used to decay the arguments, which would produce
scope guards with pointers to functions in template arguments. Although
this worked, formally only references to functions are supported. So
changed the deduction guides to produce references to functions.
Updated docs and tests.
Fixes https://github.com/Lastique/scope/issues/6.
