bug#7908: png-1.5 fix for emacs-23.2 and HEAD

[Eli Zaretskii]

> From: Chong Yidong <cyd@stupidchicken.com>
> Cc: tk@giga.or.at,  7908@debbugs.gnu.org
> Date: Sat, 29 Jan 2011 10:44:19 -0500
> 
> >   image.c: In function `png_load':
> >   image.c:5866: error: invalid type argument of `unary *'
> 
> Hmm, interesting.  That line compiles fine on GNU/Linux.  Could you
> double check if you used the second (corrected) patch I sent?  Line 5866
> should read
> 
>   if (setjmp (PNG_JMPBUF (png_ptr)))

Sorry, I used an old patch.  But with this line corrected, the
compilation still fails with the same error message.

(I'm using libpng 1.4.x here, but I modified the #ifdef conditionals
to make the new code in effect on 1.4.x.  Maybe that's the reason for
the difference?)