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

[Eli Zaretskii]

> From: Chong Yidong <cyd@stupidchicken.com>
> Cc: 7908@debbugs.gnu.org, tk@giga.or.at
> Date: Sat, 29 Jan 2011 15:33:00 -0500
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Does this alternate definition of PNG_JMPBUF make sense?
> >
> > #define PNG_JMPBUF(ptr) \
> >   (*(jmp_buf *)(fn_png_set_longjmp_fn((ptr), longjmp, sizeof (jmp_buf))))
> 
> Aha, I think it makes sense.  Since DEF_IMGLIB_FN defines a return type
> of int, whereas fn_png_set_longjmp_fn actually returns a (jmp_buf *), we
> do indeed need to perform the extra cast.  Well spotted.  Does Emacs
> compile properly with this change?

Yes, it does.