Don't specify a particular zlib version. Resolves #14.

png.mod/libpng/pnglibconf.h

@@ -210,7 +210,7 @@
 #define PNG_USER_HEIGHT_MAX 1000000
 #define PNG_USER_WIDTH_MAX 1000000
 #define PNG_ZBUF_SIZE 8192
-#define PNG_ZLIB_VERNUM 0x12c0
+#define PNG_ZLIB_VERNUM 0
 #define PNG_Z_DEFAULT_COMPRESSION (-1)
 #define PNG_Z_DEFAULT_NOFILTER_STRATEGY 0
 #define PNG_Z_DEFAULT_STRATEGY 1