Detect SDK on Darwin. Calls xcrun --sdk <target_sdk> --show-sdk-path
which fetches the path to the SDK sysroot (if any). Subsequently calls xcrun --sdk <target_sdk> --show-sdk-version
which fetches version of the SDK. The caller needs to deinit the resulting struct. stderr from xcrun is ignored. If error.OutOfMemory occurs in Allocator, this function returns null.