Finish the conditional dependencies mechanism.

Finish the conditional dependencies mechanism.

* gnulib-tool: New option --no-conditional-dependencies.
(func_usage): Document it. Don't mark --conditional-dependencies as
experimental.
(cond_dependencies): The possible values can now be true, false, empty.
(func_modules_transitive_closure, func_emit_autoconf_snippets): Update.
(func_import): Store setting in gnulib-cache.m4 and read it from there.
* doc/gnulib-tool.texi (Conditional dependencies): New section.