Fix compilation with GCC 4.8

src/checks.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 250 matching lines @@
 #define SEMI_STATIC_JOIN_HELPER(a, b) a##b
 // Causes an error during compilation of the condition is not
 // statically known to be true.  It is formulated as a typedef so that
 // it can be used wherever a typedef can be used.  Beware that this
 // actually causes each use to introduce a new defined type with a
 // name depending on the source line.
 template <int> class StaticAssertionHelper { };
 #define STATIC_CHECK(test)                                                    \
   typedef                                                                     \
     StaticAssertionHelper<sizeof(StaticAssertion<static_cast<bool>((test))>)> \
-    SEMI_STATIC_JOIN(__StaticAssertTypedef__, __LINE__)
+    SEMI_STATIC_JOIN(__StaticAssertTypedef__, __LINE__) V8_UNUSED
 #endif
 
 
 #ifdef DEBUG
 #ifndef OPTIMIZED_DEBUG
 #define ENABLE_SLOW_ASSERTS    1
 #endif
 #endif
 
 namespace v8 {
@@ skipping 40 matching lines @@
 
 // "Extra checks" are lightweight checks that are enabled in some release
 // builds.
 #ifdef ENABLE_EXTRA_CHECKS
 #define EXTRA_CHECK(condition) CHECK(condition)
 #else
 #define EXTRA_CHECK(condition) ((void) 0)
 #endif
 
 #endif  // V8_CHECKS_H_