Fix test for  use of noreturn.
diff --git a/awk.h b/awk.h
index 36a4286..5a55301 100644
--- a/awk.h
+++ b/awk.h
@@ -25,7 +25,7 @@
 #include <assert.h>
 #include <stdint.h>
 #include <stdbool.h>
-#if __STDC__ <= 199901L
+#if __STDC_VERSION__ <= 199901L
 #define noreturn
 #else
 #include <stdnoreturn.h>