Fix build with external libpng
Fix build with external libpng

olifre/root,karies/root,karies/root,olifre/root,karies/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,karies/root,karies/root,karies/root,karies/root,karies/root,olifre/root,olifre/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,root-mirror/root