please take into consideration the patch attached, which fixes a compilation error when using llvm/clang.
See http://llvm.org/bugs/show_bug.cgi?id=10338 for detailed information.
This was tested on Apple clang version 2.1 (tags/Apple/clang-163.7.1) (based on LLVM 3.0svn) Target: x86_64-apple-darwin11.3.0.
png fix for llvm/clang