The bundled zlib library will be used, so make sure we use the bundled zlib
header too to prevent build failure:

Undefined symbols for architecture x86_64:
  "_inflateValidate", referenced from:
      _png_inflate_claim in pngrutil.o
ld: symbol(s) not found for architecture x86_64

https://sourceforge.net/p/irrlicht/bugs/459/
--- source/Irrlicht/MacOSX/MacOSX.xcodeproj/project.pbxproj.orig	2013-11-17 20:11:00.000000000 -0600
+++ source/Irrlicht/MacOSX/MacOSX.xcodeproj/project.pbxproj	2023-08-01 14:05:46.000000000 -0500
@@ -6091,7 +6091,10 @@
 				ARCHS = "$(NATIVE_ARCH_ACTUAL)";
 				GCC_WARN_ABOUT_RETURN_TYPE = YES;
 				GCC_WARN_UNUSED_VARIABLE = NO;
-				HEADER_SEARCH_PATHS = ../../../include;
+				HEADER_SEARCH_PATHS = (
+					"../../../include",
+					"../zlib",
+				);
 				OTHER_CFLAGS = (
 					"-DMACOSX",
 					"-D_DEBUG",
@@ -6112,7 +6115,10 @@
 				GCC_SYMBOLS_PRIVATE_EXTERN = YES;
 				GCC_WARN_ABOUT_RETURN_TYPE = YES;
 				GCC_WARN_UNUSED_VARIABLE = NO;
-				HEADER_SEARCH_PATHS = ../../../include;
+				HEADER_SEARCH_PATHS = (
+					"../../../include",
+					"../zlib",
+				);
 				INSTALL_MODE_FLAG = "a+rwx";
 				OTHER_CFLAGS = "-DMACOSX";
 				PREBINDING = NO;