diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h
--- a/sys/sys/cdefs.h
+++ b/sys/sys/cdefs.h
@@ -349,9 +349,7 @@
 
 /*
  * Compiler-dependent macros to declare that functions take printf-like
- * or scanf-like arguments.  They are null except for versions of gcc
- * that are known to support the features properly (old versions of gcc-2
- * didn't permit keeping the keywords out of the application namespace).
+ * or scanf-like arguments.
  */
 #define	__printflike(fmtarg, firstvararg) \
 	    __attribute__((__format__ (__printf__, fmtarg, firstvararg)))
@@ -363,8 +361,20 @@
 #define	__strftimelike(fmtarg, firstvararg) \
 	    __attribute__((__format__ (__strftime__, fmtarg, firstvararg)))
 
+/*
+ * Like __printflike, but allows fmtarg to be NULL. All supported FreeBSD
+ * versions of gcc have this feature, but stock versions of gcc, including
+ * lang/gcc, do not. Clang has it for all supported versions. While stock
+ * versions of gcc ignore printf0 in system headers, we define it away so
+ * -Wsystem-headers is more useful for those versions. tcc and pcc mirror gcc's
+ * behavior.
+ */
+#if defined(__clang__) || (defined(__GNUC__) && defined(__FreeBSD_cc_version))
 #define	__printf0like(fmtarg, firstvararg) \
 	    __attribute__((__format__ (__printf0__, fmtarg, firstvararg)))
+#else
+#define	__printf0like(fmtarg, firstvararg)
+#endif
 
 #define	__strong_reference(sym,aliassym)	\
 	extern __typeof (sym) aliassym __attribute__ ((__alias__ (#sym)))