Makefile: introduce debugsymbols=on flag

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 79 matching lines @@
 ifeq ($(extrachecks), off)
   GYPFLAGS += -Ddcheck_always_on=0 -Dv8_enable_handle_zapping=0
 endif
 # slowdchecks=on/off
 ifeq ($(slowdchecks), on)
   GYPFLAGS += -Dv8_enable_slow_dchecks=1
 endif
 ifeq ($(slowdchecks), off)
   GYPFLAGS += -Dv8_enable_slow_dchecks=0
 endif
+# debugsymbols=on
+ifeq ($(debugsymbols), on)
+  GYPFLAGS += -Drelease_extra_cflags=-ggdb3
+endif
 # gdbjit=on/off
 ifeq ($(gdbjit), on)
   GYPFLAGS += -Dv8_enable_gdbjit=1
 endif
 ifeq ($(gdbjit), off)
   GYPFLAGS += -Dv8_enable_gdbjit=0
 endif
 # vtunejit=on
 ifeq ($(vtunejit), on)
   GYPFLAGS += -Dv8_enable_vtunejit=1
@@ skipping 381 matching lines @@
 # We need to manually set the stack limit here, to work around bugs in
 # gmake-3.81 and global-5.7.1 on recent 64-bit Linux systems.
 GPATH GRTAGS GSYMS GTAGS: gtags.files $(shell cat gtags.files 2> /dev/null)
         @bash -c 'ulimit -s 10240 && GTAGSFORCECPP=yes gtags -i -q -f $<'
 
 gtags.clean:
         rm -f gtags.files GPATH GRTAGS GSYMS GTAGS
 
 dependencies builddeps:
         $(error Use 'gclient sync' instead)