Add NDEBUG option to release flags so assertions are disabled in release builds. (#439)

diff --git a/ios-deploy.xcodeproj/project.pbxproj b/ios-deploy.xcodeproj/project.pbxproj
index 68a8e86..c5cae23 100644
--- a/ios-deploy.xcodeproj/project.pbxproj
+++ b/ios-deploy.xcodeproj/project.pbxproj
@@ -472,6 +472,7 @@
 				);
 				GCC_C_LANGUAGE_STANDARD = gnu99;
 				GCC_NO_COMMON_BLOCKS = YES;
+				GCC_PREPROCESSOR_DEFINITIONS = "NDEBUG=1";
 				GCC_WARN_64_TO_32_BIT_CONVERSION = YES;
 				GCC_WARN_ABOUT_RETURN_TYPE = YES_ERROR;
 				GCC_WARN_UNDECLARED_SELECTOR = YES;