Index: src/runtime/runtime-private.h
==================================================================
--- src/runtime/runtime-private.h
+++ src/runtime/runtime-private.h
@@ -15,10 +15,15 @@
  */
 
 #include "config.h"
 
 #include "platform.h"
+
+#if !defined(__has_feature) || !__has_feature(nullability)
+# define _Nonnull
+# define _Nullable
+#endif
 
 struct objc_abi_class {
 	struct objc_abi_class *_Nonnull metaclass;
 	const char *_Nullable superclass;
 	const char *_Nonnull name;
@@ -214,5 +219,10 @@
 		fprintf(stderr, "[objc @ " __FILE__ ":%d] ", __LINE__);	\
 		fprintf(stderr, __VA_ARGS__);				\
 		fputs("\n", stderr);					\
 		abort();						\
 	}
+
+#if !defined(__has_feature) || !__has_feature(nullability)
+# undef _Nonnull
+# undef _Nullable
+#endif