Currently jextract only do checking symbols if -l is specified on the command line, and it supports different options on how to deal with missing symbols via —missing-symbols.

However, one option listed, warn, is missing implementation.

With JDK-8223247, jextract now assumes we are using the default libraries within JVM if no -l option is provided. It makes sense to do symbol checking against default libraries for those case.

Propose change of jextract to:

- Check symbols remain same as current behavior if -l is specified.
- Check symbols within default libraries if no -l is provided.
- If a symbols is missing, handle accordingly based on the --missing-symbols option.
- When --missing-symbols is not provided, default is to issue warnings if a symbol is missing when -l is not specified. Exclude otherwise.

The new proposal only change behavior when -l is not specified, and it is mostly backward compatible as it doesn’t change generated code/classes, but show warnings to inform user about potential missing libraries.

Use '—missing-symbols ignore’ to suppress the warning messages if current behavior is required.