Add __printflike() from FreeBSD.

#if 0 out FreeBSD specific extension for now.

#
#
# patch "sys/sys/cdefs.h"
#  from [03c25b5f0381cc519cff842542ae176c6e8cbb16]
#    to [3050ae8e1f365db94376b79881541521a9b5107b]
#
============================================================
--- sys/sys/cdefs.h	03c25b5f0381cc519cff842542ae176c6e8cbb16
+++ sys/sys/cdefs.h	3050ae8e1f365db94376b79881541521a9b5107b
@@ -322,6 +322,34 @@
 #endif
 
 /*
+ * 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).
+ */
+#if !__GNUC_PREREQ__(2, 7) && !defined(__INTEL_COMPILER)
+#define	__printflike(fmtarg, firstvararg)
+#define	__scanflike(fmtarg, firstvararg)
+#define	__format_arg(fmtarg)
+#else
+#define	__printflike(fmtarg, firstvararg) \
+	    __attribute__((__format__ (__printf__, fmtarg, firstvararg)))
+#define	__scanflike(fmtarg, firstvararg) \
+	    __attribute__((__format__ (__scanf__, fmtarg, firstvararg)))
+#define	__format_arg(fmtarg)	__attribute__((__format_arg__ (fmtarg)))
+#endif
+
+#if 0
+/* Compiler-dependent macros that rely on FreeBSD-specific extensions. */
+#if __FreeBSD_cc_version >= 300001 && defined(__GNUC__) && !defined(__INTEL_COMPILER)
+#define	__printf0like(fmtarg, firstvararg) \
+	    __attribute__((__format__ (__printf0__, fmtarg, firstvararg)))
+#else
+#define	__printf0like(fmtarg, firstvararg)
+#endif
+#endif
+
+/*
  * Macros for manipulating "link sets".  Link sets are arrays of pointers
  * to objects, which are gathered up by the linker.
  *