list, oset, xlist, xoset: fix extern inline issue with C99

list, oset, xlist, xoset: fix extern inline issue with C99

This was introduced by my recent changes for 'inline'.
Problem reported for gettext by Daiki Ueno in
<http://lists.gnu.org/archive/html/bug-gnulib/2012-12/msg00000.html>.
* lib/gl_list.h (gl_list_nx_create_empty, gl_list_create)
(gl_list_nx_create, gl_list_size, gl_list_node_value)
(gl_list_node_set_value, gl_list_node_nx_set_value, gl_list_next_node)
(gl_list_previous_node, gl_list_get_at)
(gl_list_nx_set_at, gl_list_search, gl_list_search_from)
(gl_list_search_from_to, gl_list_indexof, gl_list_indexof_from)
(gl_list_indexof_from_to, gl_list_nx_add_first, gl_list_nx_add_last)
(gl_list_nx_add_before, gl_list_nx_add_after, gl_list_nx_add_at)
(gl_list_remove_node, gl_list_remove_at, gl_list_remove, gl_list_free)
(gl_list_iterator, gl_list_iterator_from_to, gl_list_iterator_next)
(gl_list_iterator_free, gl_sortedlist_search)
(gl_sortedlist_search_from_to, gl_sortedlist_indexof)
(gl_sortedlist_indexof_from_to, gl_sortedlist_add, gl_sortedlist_nx_add)
(gl_sortedlist_remove):
* lib/gl_oset.h (go_oset_nx_create_empty, gl_oset_size, gl_oset_search)
(gl_oset_search_atleast, gl_oset_nx_add, gl_oset_remove, gl_oset_free)
(gl_oset_iterator, gl_oset_iterator_next, gl_oset_iterator_free):
* lib/gl_xlist.h (gl_list_create_empty, gl_list_create)
(gl_list_node_set_value, gl_list_set_at, gl_list_add_first)
(gl_list_add_last, gl_list_add_before, gl_list_add_after)
(gl_list_add_at, gl_sortedlist_add):
* lib/gl_xoset.h (gl_oset_create_empty, gl_oset_add):
Wrap these extern decls inside "#if 0", because they are implemented
as inline functions, and extern inline is not what's wanted here.
It would simplify these .h files to remove the extern decls entirely,
although a downside would be less-clear separation between
specification and implementation.