maint: fix some warnings that break compilation with -Werror.

* m4/hash.c (ITERATOR_NEXT_NEXT): Hide unused macro.
(maybe_grow): Prevent implicit double promotion.
(m4_get_hash_length, m4_get_hash_iterator_key)
(m4_get_hash_iterator_value, m4_hash_string_hash)
(m4_hash_string_cmp): Add pure attribute for gcc.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>