Portable alloca for Git

In the next patch we'll have to use alloca() for performance reasons,
but since alloca is non-standardized and is not portable, let's have a
trick with compatibility wrappers:

1. at configure time, determine, do we have working alloca() through
alloca.h, and define

#define HAVE_ALLOCA_H

if yes.

2. in code

#ifdef HAVE_ALLOCA_H
# include <alloca.h>
# define xalloca(size) (alloca(size))
# define xalloca_free(p) do {} while(0)
#else
# define xalloca(size) (xmalloc(size))
# define xalloca_free(p) (free(p))
#endif

and use it like

func() {
p = xalloca(size);
...

xalloca_free(p);
}

This way, for systems, where alloca is available, we'll have optimal
on-stack allocations with fast executions. On the other hand, on
systems, where alloca is not available, this gracefully fallbacks to
xmalloc/free.

Both autoconf and config.mak.uname configurations were updated. For
autoconf, we are not bothering considering cases, when no alloca.h is
available, but alloca() works some other way - its simply alloca.h is
available and works or not, everything else is deep legacy.

For config.mak.uname, I've tried to make my almost-sure guess for where
alloca() is available, but since I only have access to Linux it is the
only change I can be sure about myself, with relevant to other changed
systems people Cc'ed.

NOTE

SunOS and Windows had explicit -DHAVE_ALLOCA_H in their configurations.
I've changed that to now-common HAVE_ALLOCA_H=YesPlease which should be
correct.

Cc: Brandon Casey <drafnel@gmail.com>
Cc: Marius Storm-Olsen <mstormo@gmail.com>
Cc: Johannes Sixt <j6t@kdbg.org>
Cc: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Cc: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Cc: Gerrit Pape <pape@smarden.org>
Cc: Petr Salinger <Petr.Salinger@seznam.cz>
Cc: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Thomas Schwinge <thomas@codesourcery.com> (GNU Hurd changes)
Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>