Fix compilation with GCC 4.8

include/v8config.h

Index: include/v8config.h
diff --git a/include/v8config.h b/include/v8config.h
index 834f9c50ec07794ee5eaf22696bc7664bca8b7a8..631ad0d0479cb915a153a57778d98a83b21460ef 100644
--- a/include/v8config.h
+++ b/include/v8config.h
@@ -187,6 +187,7 @@
 //                                        supported
 //  V8_HAS_ATTRIBUTE_DEPRECATED         - __attribute__((deprecated)) supported
 //  V8_HAS_ATTRIBUTE_NOINLINE           - __attribute__((noinline)) supported
+//  V8_HAS_ATTRIBUTE_UNUSED             - __attribute__((unused)) supported
 //  V8_HAS_ATTRIBUTE_VISIBILITY         - __attribute__((visibility)) supported
 //  V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT - __attribute__((warn_unused_result))
 //                                        supported
@@ -216,6 +217,7 @@
 # define V8_HAS_ATTRIBUTE_ALWAYS_INLINE (__has_attribute(always_inline))
 # define V8_HAS_ATTRIBUTE_DEPRECATED (__has_attribute(deprecated))
 # define V8_HAS_ATTRIBUTE_NOINLINE (__has_attribute(noinline))
+# define V8_HAS_ATTRIBUTE_UNUSED (__has_attribute(unused))
 # define V8_HAS_ATTRIBUTE_VISIBILITY (__has_attribute(visibility))
 # define V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT \
     (__has_attribute(warn_unused_result))
@@ -247,6 +249,7 @@
 # define V8_HAS_ATTRIBUTE_DEPRECATED (V8_GNUC_PREREQ(3, 4, 0))
 # define V8_HAS_ATTRIBUTE_DEPRECATED_MESSAGE (V8_GNUC_PREREQ(4, 5, 0))
 # define V8_HAS_ATTRIBUTE_NOINLINE (V8_GNUC_PREREQ(3, 4, 0))
+# define V8_HAS_ATTRIBUTE_UNUSED (V8_GNUC_PREREQ(2, 95, 0))
 # define V8_HAS_ATTRIBUTE_VISIBILITY (V8_GNUC_PREREQ(4, 3, 0))
 # define V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT \
     (!V8_CC_INTEL && V8_GNUC_PREREQ(4, 1, 0))
@@ -334,6 +337,14 @@ declarator __attribute__((deprecated))
 #endif
 
 
+// A macro to mark variables or types as unused, avoiding compiler warnings.
+#if V8_HAS_ATTRIBUTE_UNUSED
+# define V8_UNUSED __attribute__((unused))
+#else
+# define V8_UNUSED
+#endif
+
+
 // Annotate a function indicating the caller must examine the return value.
 // Use like:
 //   int foo() V8_WARN_UNUSED_RESULT;