Support linux build

[paulcadman]

This PR parametrises:

1. The lakefile. The macOS specific framework and isysroot flags are set only when run on macOS platform
2. The Raylib build. I'm setting the compiler to GCC because that's what Raylib does in its linux build.

I've added a Github Action that builds the project on linux.

Dependencies

To get the build working I installed:

sudo apt-get install -y --no-install-recommends libglfw3 libglfw3-dev libx11-dev libxcursor-dev libxrandr-dev libxinerama-dev libxi-dev libxext-dev libxfixes-dev libwayland-dev libxkbcommon-dev

This is probably not minimal, is taken from the raylib linux build workflow.

Testing

I've only tested that the project builds on linux. I only have headless linux so I can't run it.

[awalterschulze]

@keeganperry7 tested this on ubuntu and it works :D