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

[Eli Zaretskii]

> Date: Mon, 24 Jan 2011 13:54:37 +0100
> From: Thomas Klausner <tk@giga.or.at>
> Cc: 
> 
> emacs-23.2 and git master from a few minutes don't compile against the
> rather new png-1.5, which is hiding internal structure members.

"Doesn't compile" is a bit of exaggeration, I think: the members are
declared with `__attribute__((deprecated))', which produces warning
messages.

> The attached patch fixes the problem. Please include it.

Thanks, but this patch won't work with dynamic link against libpng, at
least not on MS-Windows, because the expansion of png_jmpbuf, viz.:

  #  define png_jmpbuf(png_ptr) \
     (*png_set_longjmp_fn((png_ptr), longjmp, sizeof (jmp_buf)))

causes link errors, since png_set_longjmp_fn is not available (a NULL
pointer) until the DLL is loaded.