Fix noreturn for old compilers.
diff --git a/awk.h b/awk.h
index 4e01325..36a4286 100644
--- a/awk.h
+++ b/awk.h
@@ -25,7 +25,11 @@
 #include <assert.h>
 #include <stdint.h>
 #include <stdbool.h>
+#if __STDC__ <= 199901L
+#define noreturn
+#else
 #include <stdnoreturn.h>
+#endif
 
 typedef double	Awkfloat;