While at it, add comment to indicate what the version signifies. [a3f: updated commit message]
So user code can use add_subdirectory to build it (similar to what we do with GLFW or what the projects/CMake/CMakeLists.txt can do).
The CMakeLists.txt checks for an installed raylib and downloads and installs one if none is found. Afterwards, it builds core_basic_window.c