Define NDEBUG in release Xcode builds

Xcode/SDL/SDL.xcodeproj/project.pbxproj

@@ -3051,7 +3051,10 @@
 				GCC_GENERATE_DEBUGGING_SYMBOLS = NO;
 				GCC_NO_COMMON_BLOCKS = YES;
 				GCC_OPTIMIZATION_LEVEL = 3;
-				GCC_PREPROCESSOR_DEFINITIONS = "$(CONFIG_PREPROCESSOR_DEFINITIONS)";
+				GCC_PREPROCESSOR_DEFINITIONS = (
+					"$(CONFIG_PREPROCESSOR_DEFINITIONS)",
+					NDEBUG,
+				);
 				GCC_SYMBOLS_PRIVATE_EXTERN = YES;
 				HEADER_SEARCH_PATHS = (
 					../../include,