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

[Chong Yidong]

Eli Zaretskii <eliz@gnu.org> writes:

> 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))))

Aha, I think it makes sense.  Since DEF_IMGLIB_FN defines a return type
of int, whereas fn_png_set_longjmp_fn actually returns a (jmp_buf *), we
do indeed need to perform the extra cast.  Well spotted.  Does Emacs
compile properly with this change?