Index: include/stddef.h
===================================================================
--- include/stddef.h
+++ include/stddef.h
@@ -70,7 +70,10 @@
 #endif
 #endif
 
+#ifndef _OFFSETOF_DECLARED
 #define	offsetof(type, member)	__offsetof(type, member)
+#define	_OFFSETOF_DECLARED
+#endif
 
 #if __EXT1_VISIBLE
 /* ISO/IEC 9899:2011 K.3.3.2 */
Index: sys/sys/stddef.h
===================================================================
--- sys/sys/stddef.h
+++ sys/sys/stddef.h
@@ -38,6 +38,9 @@
 #define	_PTRDIFF_T_DECLARED
 #endif
 
+#ifndef _OFFSETOF_DECLARED
 #define	offsetof(type, field)	__offsetof(type, field)
+#define _OFFSETOF_DECLARED
+#endif
 
 #endif /* !_SYS_STDDEF_H_ */