cmake: Various macOS build/package improvements

This properly links dylibs set during build instead of expecting the libraries to be in the system path.

In the wiki there is a guide that recommends installing compiled dependencies to /usr/local/lib and adding that to the linker path. I don't really think that's the best way to handle this; I'm not sure why I wrote the CMakeLists.txt to link this way originally, but this is the correct way to do it as it should support both dylib linkage and framework linkage for SDL2 and SDL2_mixer.

Now we also fix the LC_RPATH issues when attempting to install or package on recent versions of macOS too!

Finally, some smaller changes to make the bundle look nicer (setting the icon, version, and bundle name)