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.
f2d91cde80