CMake improvements

[abelfodil]

Problems:

• To add Boost.DI to a target, one has to execute target_include_directories(target_name PRIVATE path/to/boost/di/include)
• It isn't recommended to use global scoped functions like include_directories (see https://gist.github.com/mbinna/c61dbb39bca0e4fb7d1f73b0d66a4fd1)
• Examples and tests pollute CLion target selection list when it isn't even used as the top project

Solutions:

• Modern CMake practices allow one to use Boost.DI like so:

  add_subdirectory(path/to/boost/di)
  target_link_libraries(target_name PRIVATE Boost.DI) # yes you can add it like a normal lib :D

• Hide tests and examples when not needed (ie when Boost.DI is included into another project)

[krzysztof-jusiak]

Awesome, thank you @abelfodil