#3 WIP: macOS support via GLFW3

**This is a work-in-progress branch, please do not merge yet :)** Adds initial support for building LaGUI on macOS, using GLFW3 as its backend since Cocoa is a bit strange. `clang` (as provided by XCode) is a bit more strict when it comes to compiling C code, so I had to add several more function declarations and fix up some `return`s. Current state: it builds fine, but I have still to figure out how to get CMake to link to a project so I can actually try it out.