build: disable v8 snapshots

Snapshots speed up start-up by a few milliseconds but are potentially
dangerous because of the fixed hash seed that is used for strings and
dictionaries, making collision denial-of-service attacks possible.

Release builds on iojs.org have snapshots disabled but source builds
did not, until now.

The risk for individual source builds is low; the binary gets a random
32 bits hash seed that should be hard to guess by an external attacker.

It's when binaries are distributed by, for example, a distro vendor
that the fixed hash seed becomes a vulnerability, because then it's
possible to target a large group of people at once.

People that really need the faster start-up time can use the new
--with-snapshot configure flag.

PR-URL: https://github.com/iojs/io.js/pull/585


Reviewed-By: Bert Belder <bertbelder@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Rod Vagg <rod@vagg.org>