build: add --enable-d8 configure option

Add an option to the configure script for building d8.  Useful for
testing V8 standalone.

PR-URL: https://github.com/nodejs/node/pull/7538


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>