System Initiative

- Run `make` to compile the `xcode_version_checker` binary. - Run `make all` to compile the `xcode_version_checker` binary and run all tests. - Run `make test` to just run tests. - Run `make clean` to delete tests and intermediate binaries. We cannot include this as part of the toolchain because of bootstrapping issues: i.e., compiling it requires an `apple_toolchain` but its needed to define an `apple_toolchain`.