#5 won't build with libpng 1.5

[Andrew E. Schulman]

Compilation of sng currently fails in Cygwin, with multiple errors like this:

/cygdrive/c/Users/andrex/Home/dev/cygwin/sng/sng-1.0.5-1/src/sng-1.0.5/sngc.c:545:23: error: dereferencing pointer to incomplete type
  if (width != info_ptr->width && height != info_ptr->height)
                       ^

and

/cygdrive/c/Users/andrex/Home/dev/cygwin/sng/sng-1.0.5-1/src/sng-1.0.5/sngd.c:953:38: error: ‘PNG_HAVE_IDAT’ undeclared (first use in this function)
  if (after_idat != !!(up->location & PNG_HAVE_IDAT))
                                      ^

These appear to be caused by dependencies on libpng 1.2, which seems to be deprecated. Anyway libpng 1.2 is no longer available in Cygwin, so sng will no longer build there.

[Max Horn]

I made a version of sng that works with current libpng, you can find its code here: https://github.com/fingolfin/sng/tree/modern-libpng

I also emailed esr about it, so perhaps this can be integrated into sng soon.