Add dependency on Config to check targets

[grisumbras]

The targets used for config checks do not have a dependency on /boost/config//boost_config. This is not a problem when the files are used as a part of the actual Boost.Config project. But consider this use case:

1. Boost.Config is installed on the user's machine, including its headers and b2 extension modules.
2. A Jam project that represents /boost/config is generated for the user.
3. A -config.jam file is generated for the user that points to the generated project via

   project-search /boost/config : path/to/boost/config/installation ;
   

4. User's own jam files use Boost.Config's checks e.g.

   import-search /boost/config/checks ;
   import config ;
   
   lib mylib : mylib.cpp : [ config.requires cxx11_constexpr ] ;
   

In this case Boost.Config's build.jam is not used and so, the check targets cannot find the necessary headers.

This use case is how Conan package manager works. So, the change is aimed to provide support for modular build to Boost libraries using Conan.