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

[Eli Zaretskii]

> Date: Sat, 29 Jan 2011 21:02:09 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 7908@debbugs.gnu.org, tk@giga.or.at
> 
> There's no Windows port of libpng 1.5 yet, but I will at least try
> compiling with the headers from there.

Tried that now.  Same error message.

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))))