Make AC_PREREQ faster and more robust.

Make AC_PREREQ faster and more robust.

* lib/m4sugar/m4sugar.m4 (m4_ignore, m4_unquote): New macros.
(m4_version_prereq): Inline constant expansions.
(m4_list_cmp): Reduce number of expansions, by avoiding m4_case.
Rewrite in terms of [] list, not () list.
(_m4_list_cmp, _m4_version_unletter): New helper macros.
(m4_version_unletter): Write wrapper around new implementation to
preserve old semantics.
(m4_version_compare): Pass correct type of list, and avoid
overhead of flattening expressions too early.
(m4_do): Move to be near other quoting macros.
(m4_max, m4_min): Always result in decimal output.
* doc/autoconf.texi (Looping constructs): Add m4_car, m4_cdr.
Move m4_do...
(Evaluation Macros): ...here.  Add m4_ignore, m4_unquote.
(Text processing Macros): Move m4_version_compare...
(Number processing Macros): ...to this new node; document m4_cmp,
m4_list_cmp, m4_sign, m4_max, m4_min.
* tests/m4sugar.at (m4@&t@_version_compare): Enhance test, to pick
up on bugs fixed by this patch.
* NEWS: Document new macros.

Signed-off-by: Eric Blake <ebb9@byu.net>