Provide better short-circuiting operation.

Provide better short-circuiting operation.

* lib/m4sugar/m4sugar.m4 (m4_cond, m4_newline): New macros.
(m4_text_wrap): Use it.  Also avoid useless m4_for.
* lib/m4sugar/m4sh.m4 (_AS_QUOTE_IFELSE, AS_LITERAL_IF): Use
new macro.
(_AS_IDENTIFIER_IF): Likewise, and fix bug when $1 is [,].
* lib/autotest/general.m4 (_AT_DECIDE_TRACEABLE): Use new macros
to avoid regexps.
* doc/autoconf.texi (Redefined M4 Macros): Expand m4_if
documentation.  Sort m4_mkstemp, m4_undefine.  Move m4_ifndef...
(Conditional constructs): ...here, to new section.  Also document
m4_cond, m4_ifval, m4_n, m4_ifvaln, m4_ifset, m4_case, m4_bmatch,
m4_bpatsubsts, and m4_default.
(Looping constructs): Document m4_shiftn, m4_shift2, m4_shift3,
m4_do.

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