GD Won't Complie Against libpng 1.4.0

From the libpng README.

"Removed the deprecated png_check_sig() function/macro."

The file ext/gd/libgd/gd_png.c currently uses this function and so will not compile against libpng 1.4.0. The libpng man page notes the following:

"The function png_check_sig(sig, num) was replaced with !png_sig_cmp(sig, 0, num). It has been deprecated since libpng-0.90."

Hehe