not working right -- will fix in subsequent commit.

Yay compatibility and predictability!

Pending:

- Tag BoringSSL repo.
- Tag gRPC repo.
- Build the plugin, zip it, and add it to the release.
- Push Cocoapods to trunk.

This works around the Cocoapods' linter header_mapping_dir restrictions.
Incidentally, it also removes the need to set build settings in the user
target.