gnulib-tool: work with NetBSD /bin/sh

gnulib-tool: work with NetBSD /bin/sh

* gnulib-tool (func_cache_var, func_cache_lookup_module)
(func_get_description, func_get_comment, func_get_status)
(func_get_notice, func_get_applicability, func_get_filelist)
(func_get_dependencies, func_get_autoconf_early_snippet)
(func_get_autoconf_snippet, func_get_automake_snippet)
(func_get_include_directive, func_get_link_directive)
(func_get_license, func_get_maintainer, func_import): Avoid
shell syntax errors from parsing syntax extensions.

Signed-off-by: Eric Blake <eblake@redhat.com>