Use SDK version as a soft default for minimum OS version

Bazel currently has arbitrary hardcoded minimum OS versions for each platform.

Once #43 is resolved, we should stop relying on the `--{i,tv,watch}os_minimum_os` flags entirely and use the SDK version as the soft default instead. It will print a warning notifying users that they are using this soft default and that they should make the minimum version explicit if they want predictable builds. (By not making the attribute mandatory, we replicate Xcode's defaults and we don't penalize the use case of quickly whipping up a BUILD target for experimentation/prototypes.)