macOS ships with several stubs in `/usr/bin` that invoke `xcrun` to run
the tools from the active SDK. When `/usr/bin` is in `PATH`, this will
cause `xcrun` from xcbuild to invoke itself over and over. Filtering
`/usr/bin` from `xcrun`’s search path prevents this from happening.
xcbuild determines the SDK dynamically, so overriding the `sdkVer` no
longer works. If packages want to change the SDK, they need to add one
of the SDK packages to their inputs.
Take advantage of the new Darwin SDKs to dynamically determine SDK
information such as path, version, and binaries (via `xcrun --find`).
This is accomplished by relying on the existance of `DEVELOPER_DIR`,
which the SDK will set up in nixpkgs.
