0.4.0

• This release is tested with Bazel 0.19.0.
• Minor debugging improvements.
• Support batch mode compilation for Xcode 10 or higher (macOS only at this time).
• Propagate simple defines between Swift and Objective-C targets.
• Ensure that transitive headers are available when a swift_library depends on a cc_library, and that their include paths are correct.
• A number of compatibility and cleanup fixes for more recent Bazel versions (thanks @keith and @kastiglione!)