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

[Chong Yidong]

Eli Zaretskii <eliz@gnu.org> writes:

> 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?)

Yeah, it makes a big difference.  png_set_longjmp_fn is new to
libpng-1.5, it's the new non-backward compatible way to access the png
jmpbuf.

If it's too much trouble for you to install libpng 1.5 on Windows, I
think I'll go ahead and commit the patch.  The way the conditionals are
now written, compilation with libpng 1.4 is unaffected, so it should be
very safe.