diff --git a/dbm/include/cdefs.h b/dbm/include/cdefs.h
index 96fa6656d33..ff6c03fabac 100644
--- a/dbm/include/cdefs.h
+++ b/dbm/include/cdefs.h
@@ -88,7 +88,9 @@
 #define	__CONCAT(x,y)	x ## y
 #define	__STRING(x)	#x
 
+#ifndef __const
 #define	__const		const		/* define reserved names to standard */
+#endif  /* __const */
 #define	__signed	signed
 #define	__volatile	volatile
 #ifndef _WINDOWS