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

[Eli Zaretskii]

> From: Chong Yidong <cyd@stupidchicken.com>
> Cc: Thomas Klausner <tk@giga.or.at>, 7908@debbugs.gnu.org
> Date: Tue, 25 Jan 2011 15:57:38 -0500
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> 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.
> 
> Do you have the exact link errors?  If possible, please try with this
> patch (the libpng API changes are a bit of a mess, and it's possible
> that small changes might affect success or failure).

Will do.  Do you plan this for 23.3, or just for the trunk?  If the
latter, I'd like to unbreak the Windows build on the trunk first.