Support for conditional dependencies.

Support for conditional dependencies.

* doc/gnulib.texi (Module description): Document the syntax of
conditional dependencies.
* gnulib-tool: New option --conditional-dependencies.
(func_usage): Document it.
(cond_dependencies): New variable.
(func_get_automake_snippet_conditional,
func_get_automake_snippet_unconditional): New functions, extracted from
func_get_automake_snippet.
(func_get_automake_snippet): Use them.
(sed_first_32_chars): New variable.
(func_module_shellfunc_name): New function.
(func_module_shellvar_name): New function.
(func_module_conditional_name): New function.
(func_uncond_add_module, func_conddep_add_module, func_cond_module_p,
func_cond_module_condition): New functions.
(func_modules_transitive_closure): Add support for conditional
dependencies.
(func_emit_lib_Makefile_am): For a conditional module, enclose the
conditional automake snippet in an automake conditional.
(func_emit_autoconf_snippets): Emit shell functions that contain the
code for conditional modules.
(func_import, func_create_testdir): Update specification.