Some dependencies are wrongly treated by CMake as optional instead of required

[krzys-h]

Instead of emitting missing library errors like all the others do, they fail with an error like this:

CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
GLEW_INCLUDE_PATH
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
GLEW_LIBRARY
    linked by target "colobot" in directory /home/krzys_h/colobot/src
PHYSFS_INCLUDE_PATH
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
   used as include directory in directory /home/krzys_h/colobot/src
PHYSFS_LIBRARY
    linked by target "colobot" in directory /home/krzys_h/colobot/src