Add build system infrastructure for ENABLE_EXTRA_CHECKS flag (not used yet)

Makefile

 # 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 44 matching lines @@
   GYPFLAGS += -Dv8_enable_disassembler=1
 endif
 # objectprint=on
 ifeq ($(objectprint), on)
   GYPFLAGS += -Dv8_object_print=1
 endif
 # snapshot=off
 ifeq ($(snapshot), off)
   GYPFLAGS += -Dv8_use_snapshot='false'
 endif
+# extrachecks=on/off
+ifeq ($(extrachecks), on)
+  GYPFLAGS += -Dv8_enable_extra_checks=1
+endif
+ifeq ($(extrachecks), off)
+  GYPFLAGS += -Dv8_enable_extra_checks=0
+endif
 # gdbjit=on
 ifeq ($(gdbjit), on)
   GYPFLAGS += -Dv8_enable_gdbjit=1
 endif
 # liveobjectlist=on
 ifeq ($(liveobjectlist), on)
   GYPFLAGS += -Dv8_use_liveobjectlist=true
 endif
 # vfp3=off
 ifeq ($(vfp3), off)
@@ skipping 194 matching lines @@
 
 # Stores current GYPFLAGS in a file.
 $(ENVFILE).new:
         @mkdir -p $(OUTDIR); echo "GYPFLAGS=$(GYPFLAGS)" > $(ENVFILE).new; \
             echo "CXX=$(CXX)" >> $(ENVFILE).new
 
 # Dependencies.
 dependencies:
         svn checkout --force http://gyp.googlecode.com/svn/trunk build/gyp \
             --revision 1282