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

[Eli Zaretskii]

> Date: Mon, 24 Jan 2011 22:59:40 +0100
> From: Thomas Klausner <tk@giga.or.at>
> Cc: 7908@debbugs.gnu.org
> 
> DEF_IMGLIB_FN (png_set_longjmp_fn);
> ...
> LOAD_IMGLIB_FN (library, png_set_longjmp_fn);
> ...
> #define fn_png_set_longjmp_fn png_set_longjmp_fn
> ...
> setjmp(*png_set_longjmp_fn(png_ptr, longjmp, sizeof(jmp_buf)));
> ...
> 
> and similarly for png_longjmp. Completely untested, just guessed,
> since I haven't tried compiling emacs on MS-Windows before.

This doesn't work, I tried that a few weeks ago, and I still get link
errors.