Request `-java-lib-external`

Feature request for link **"external"** (or "runtime"?) JARs who have not to be copied to build-dir/lib/ and not be mentioned in `manifest`.

Start by locating the handling of `-java-lib` and the build logic that copies JARs into `build-dir/lib/` or adds them to the `manifest`. Clarify whether the requested option should mean external or runtime libraries, then define the expected behavior for each before implementing and testing the change.

索引モデルが issue の本文から書いたものです。