* gnulib-tool (func_usage): Document the new options
--extract-recursive-dependencies, --extract-recursive-link-directive.
(func_verify_module): Document output variables.
(func_get_dependencies_recursively): New function.
(func_get_link_directive_recursively): New function.
Use them to implement the new options
--extract-recursive-dependencies, --extract-recursive-link-directive.
* doc/gnulib-tool.texi (Link-time requirements): New section.