We currently use CMake 3.16 for compatibility with Ubuntu 18.04 LTS.
The `NAME_MISMATCHED` argument which turns off a CMake warning
when using `FIND_PACKAGE_HANDLE_STANDARD_ARGS`, is not available
in CMake 3.16 and can prevent a successful build.
See
https://github.com/colobot/colobot/pull/1478#issuecomment-986204233
Make them export targets instead of variables.
Some platforms need libintl, abstract it to a Find*.cmake module.
Static linking requires linking to dependencies of dependencies (it is
as horrific as it sounds). This commit handles this only for libsndfile.
SDL2_image and SDL2_ttf probably also have some hidden dependencies.
FindGLEW module is now bundled with CMake, hence it is deleted.
Added Copyright.txt since some modules are taken from CMake.
Tomasz Kapuściński