alloca: port to compilers that can optimize like GCC 4.6.0

alloca: port to compilers that can optimize like GCC 4.6.0

* lib/alloca.c (find_stack_direction): New signature, taken from
Autoconf git.  This works with GCC 4.6.0.  This code should never
be used with GCC 4.6.0 itself, as GCC has alloca, but it might
be used with other compilers that optimize as well as GCC 4.6.0 does.
(alloca): Adjust to new signature.
* m4/alloca.m4 (__AC_LIBOBJ_ALLOCA) [Autoconf version < 2.69]:
New macro, which patches Autoconf in a similar way.