Add -Wall to compile. Fix one (and only) warning

ext/v8/extconf.rb

@@ -4,6 +4,8 @@ require 'mkmf'
 dir_config('v8')
 have_library('v8') or raise "unable to find libv8"
 
+$CPPFLAGS += " -Wall" unless $CPPFLAGS.split.include? "-Wall"
+
 create_makefile('v8')
 
 # now add a few extra targets

ext/v8/v8_standalone.cpp

@@ -20,7 +20,6 @@ bool is_function(VALUE& object) {
  * Debugging aid
  */
 VALUE v8_what_is_this(VALUE self, VALUE object) {
-  VALUE boolean;
   switch (TYPE(object)) {
     case T_NIL:		
     printf("nil\n");