Makes GEL more consumable as a C++ dependency via: cmake and definition as a nix package (flake)

[kdmarrett]

Commented out the line: add_subdirectory(${glfw_SOURCE_DIR} ... since I could not figure out the purpose.

[janba]

Thanks a lot for working on ways to build GEL. On one hand, I think this is a great idea, but I am a little concerned about putting these nix files in the root directory. With the CMake build files I wonder if the stuff in GEL_UNIX is really needed anymore. Maybe that stuff could be cleaned out and the nix files could go there - possibly along with other package management build recipes?

[kdmarrett]

I looked into the concern of polluting the git base directory with excessive *.nix files. It's best practice to keep the flake.nix and gel.nix files in the root directory so that they are directly findable by downstream librarires, and so users can install the library from the command line from the root directory of your project. The shell.nix files and default.nix aren't strictly necessary though and are for developer support for those who work with your library in a nix virtual environment--I could remove them. How would you like to proceed?