Defining __unused causes breakage in system headers (see #522773 and #522774).

Note: special care should be taken with flex.skl; this code isn't compiled in
lex itself, it is used as template to generate code that will later be built
in other packages (e.g. kfreebsd-9).

--- a/src/contrib/flex/flex.skl
+++ b/src/contrib/flex/flex.skl
@@ -202,8 +202,10 @@
 #include <sys/cdefs.h>
 #include <stdint.h>
 #else
+#ifndef __dead2
 #define	__dead2
 #endif
+#endif
 
 /* begin standard C headers. */
 %if-c-only