Fix CMake warnings concerning dlib_{INCLUDE_DIRS,LIBRARIES}

[ptosi]

Silence the following CMake warnings:

CMake Warning at /usr/lib/x86_64-linux-gnu/cmake/dlib/dlibConfig.cmake:42 (message):
  The variable 'dlib_INCLUDE_DIRS' is deprecated! Instead, simply use
  target_link_libraries(your_app dlib::dlib).  See
  http://dlib.net/examples/CMakeLists.txt.html for an example.

and

CMake Warning at /usr/lib/x86_64-linux-gnu/cmake/dlib/dlibConfig.cmake:42 (message):
  The variable 'dlib_LIBRARIES' is deprecated! Instead, simply use
  target_link_libraries(your_app dlib::dlib).  See
  http://dlib.net/examples/CMakeLists.txt.html for an example.

[ptosi]

Note: this might actually duplicate #22 ...

[severin-lemaignan]

Merged, thanks!