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/usr.bin/lex/flex.skl
+++ b/src/usr.bin/lex/flex.skl
@@ -5,10 +5,8 @@
  * $FreeBSD$
  */
 
-#if defined(__FreeBSD__)
 #include <sys/cdefs.h>
-#else
-#define __unused
+#ifndef __dead2
 #define __dead2
 #endif
 
@@ -264,7 +262,7 @@
 %*
 
 static void *yy_flex_alloc YY_PROTO(( yy_size_t ));
-static void *yy_flex_realloc YY_PROTO(( void *, yy_size_t )) __unused;
+static void *yy_flex_realloc YY_PROTO(( void *, yy_size_t )) __attribute__((unused));
 static void yy_flex_free YY_PROTO(( void * ));
 
 #define yy_new_buffer yy_create_buffer