bug#7716: 23.2.91; Pretest Windows binary doesn't support PNG images

[Eli Zaretskii]

> From: Jason Rumney <jasonr@gnu.org>
> Cc: bug-gnu-emacs@gnu.org
> Date: Thu, 30 Dec 2010 23:58:46 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> Faced with a choice between 1.4 and an old 1.2 with known security
> vulnerabilities, I know which one I'd rather recommend to users.

I agree, but I don't want to force users who build their own Emacs to
upgrade to a newer libpng just to have PNG support in Emacs.  So I
installed (on the emacs-23 branch) a set of changes that expose to
Lisp the version of libpng against which Emacs was compiled, and then
w32-win.el sets up the PNG part of image-library-alist
(dynamic-library-alist in Emacs 24) as appropriate for what was used
during compilation.

While at that, I also fixed image.c to avoid a compiler warning due to
direct access to png_ptr->jmpbuf.