Disable the APNG code if the version of libpng is greater than 1.4.

Since supposedly the current code *can't* work with newer versions of libpng that have the apng patch, it won't compile if that's what people have.
It seems like some distros are adding the apng patch to libpng and that'll make things annoying to compile for people.
I think it goes without saying, but if there's ever a fix to make newer libpng versions work with the APNG code then this can be reverted.