AC_FUNC_ALLOCA: port to recent BSDs and remove obsolete AIX

AC_FUNC_ALLOCA: port to recent BSDs and remove obsolete AIX

* doc/autoconf.texi (Particular Functions): Remove the AIX case
from the recommended code, as the most recent version of the AIX
compiler that IBM still supports (V10.1 as of this writing) has
<alloca.h> and thus longer needs this, and the old suggestion
wasn't completely working anyway.  Remove obsolete discussion of
SVR3 libPW alloca and of SVR4 libucb alloca.
* lib/autoconf/functions.m4 (AC_FUNC_ALLOCA):
Rework to match documentation, including abovementioned AIX change.
Inconsistency with documentation reported by Steven G. Johnson in
<http://lists.gnu.org/archive/html/autoconf/2003-03/msg00179.html>.
As this adds stdlib.h, it should also fix the problems on recent
BSD platforms noted by Patrick Welche in
http://lists.gnu.org/archive/html/autoconf-patches/2012-12/msg00009.html
though the fix differs from NetBSD's current workaround.
Also, don't bother checking for alloca if <alloca.h> works,
as the latter implies the former.