Fix libpng build for iOS

Ugh, the patch for libpng is silly, but I didn't bother now fixing the
silliness, just added the case for iOS, too.

libpng/libpng-1.5.1.patch

@@ -233,16 +233,20 @@
 +#endif /* PNGLCONF_H */
 --- misc/libpng-1.5.1/pngstruct.h	2011-02-03 05:58:17.000000000 +0100
 +++ misc/build/libpng-1.5.1/pngstruct.h	2011-03-24 21:47:23.380397048 +0100
-@@ -24,7 +24,11 @@
+@@ -24,7 +24,15 @@
   * in this structure and is required for decompressing the LZ compressed
   * data in PNG files.
   */
-+#if defined MACOSX
++/* WTF, surely this should depend explicitly on whether using the system or
++ * internal zlib, not on OS... But yeah, so maybe MacOSX (and iOS) are the only OSes for which
++ * we use a system zlib but internal libpng, at least by default.
++ */
++#if defined MACOSX || defined IOS
  #include "zlib.h"
 +#else
 +#include <external/zlib/zlib.h>
 +#endif
-
+ 
  struct png_struct_def
  {
 --- misc/libpng-1.5.1/pngtest.c	2011-02-03 05:58:17.000000000 +0100
@@ -251,7 +255,7 @@
   * of files at once by typing "pngtest -m file1.png file2.png ..."
   */
 
-+#if defined MACOSX
++#if defined MACOSX || defined IOS
  #include "zlib.h"
 +#else
 +#include <external/zlib/zlib.h>
@@ -267,7 +271,7 @@
  #endif
 
 -#include "zlib.h"   /* For crc32 */
-+#if defined MACOSX
++#if defined MACOSX || defined IOS
 +#include "zlib.h"
 +#else
 +#include <external/zlib/zlib.h>