Fixing a bug that caused cmake to set off wrong warnings about missing dependencies
Make the CMakeLists.txt smarter:
If the dependencies are found, no targets to install those are created.

Drawbacks:

[*] The paths are not set automatically. If you have Boost and SFML installed in extlibs, they will not be found. Invoke CMake like this: `cmake -DSFML_ROOT=extlibs/SFML -DBOOST_ROOT=extlibs/Boost` to find your dependencies.
[*] If the user has the dependencies installed wrongly or partial, cmake will not offer the correct installation of the libs. Invoke CMake like this: `cmake -DFORCE_EXTERNALS=1` to make CMake create targets for the dependencies nevertheless.