build: fix up build after v8 upgrade

* Define and disable new flag `v8_use_external_startup_data`.

* Disable v8_postmortem_support, it's broken again.  Fedor (@indutny)
  has volunteered to fix it up in the near future.

PR-URL: https://github.com/iojs/io.js/pull/243


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>