Improved linking method for png++ based on https://github.com/Jesse-V/iVoxely/commit/36087f1e2e41a4943450eb95028e146588aaba09#commitcomment-3577679.
Improved png++ linking technique based on suggestion https://github.com/Jesse-V/iVoxely/commit/36087f1e2e41a4943450eb95028e146588aaba09#commitcomment-3577679.