Generate .dylib on OSX

[notchris]

I was having some issues generate a .dylib file for sdl_gpu (required if using the bindings for nim). Hopefully these steps can help someone else (sorry if this is the wrong place to post this)

Pull repository

1. cd sdl_gpu
2. cmake -G "Unix Makefiles" -DCMAKE_INSTALL_PREFIX=~/Desktop/test -DBUILD_SHARED=ON
3. make
4. sudo make install
5. clang -fpic -shared -Wl,-all_load -lglfw -lSDL2 test/lib/libSDL2_gpu.a -framework OpenGL -o libSDL2_gpu.dylib
6. Move the generated libSDL2_gpu.dylib to usr/local/lib
7. If you are missing other sdl2 packages like sdl2_mixer, those are available on homebrew.