bug#12446: port better to POSIX hosts lacking _setjmp

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 14 Sep 2012 12:44:37 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: Eli Zaretskii <eliz@gnu.org>, Juanma Barranquero <lekktu@gmail.com>
> 
> +	Port better to POSIX hosts lacking _setjmp.
> +	* lisp.h: Include <setjmp.h> here, since we use its symbols here.
> +	All instances of '#include <setjmp.h>' removed, if the
> +	only reason for the instance was because "lisp.h" was included.
> +	(sys_jmp_buf, sys_setjmp, sys_longjmp): New symbols.
> +	Unless otherwise specified, replace all uses of jmp_buf, _setjmp,
> +	and _longjmp with the new symbols.  This ports better to POSIX hosts
> +	that have sigsetjmp but not _setjmp, such as some versions of
> +	Solaris and Unixware.  (Also, POSIX-2008 marks _setjmp as obsolescent.)
> +	* image.c (_setjmp, _longjmp) [HAVE_PNG && !HAVE__SETJMP]: New macros.
> +	(png_load_body) [HAVE_PNG]:
> +	(PNG_LONGJMP) [HAVE_PNG && PNG_LIBPNG_VER < 10500]:
> +	(PNG_JMPBUF) [HAVE_PNG && PNG_LIBPNG_VER >= 10500]:
> +	Use _setjmp and _longjmp rather than sys_setjmp and sys_longjmp,
> +	since PNG requires jmp_buf.  This is the only exception to the
> +	general rule that we now use sys_setjmp and sys_longjmp.
> +	This exception is OK since this code does not change the signal
> +	mask or longjmp out of a signal handler.

I don't understand the rationale.  Why not simply switch to using
ANSI- and Posix-standard setjmp/longjmp?  You are not using the
signal-mask preserving feature of sigsetjmp, so what's the point of
using it in preference to setjmp?