diff --git a/block_to_imp.c b/block_to_imp.c
index 9e0e639..9fadea2 100644
--- a/block_to_imp.c
+++ b/block_to_imp.c
@@ -17,6 +17,9 @@
 #include "lock.h"
 #include "visibility.h"
 
+#ifndef __has_builtin
+#define __has_builtin(x) 0
+#endif
 #if __has_builtin(__builtin___clear_cache)
 #	define clear_cache __builtin___clear_cache
 #else