--- libpng-1.6.21.old/png.h
+++ libpng-1.6.21/png.h
@@ -270,6 +270,8 @@
 #ifndef PNG_H
 #define PNG_H
 
+#include <zlib.h>
+
 /* This is not the place to learn how to use libpng. The file libpng-manual.txt
  * describes how to use libpng, and the file example.c summarizes it
  * with some code on which to build.  This file is useful for looking