build: Expose xz compression level

On machines with limited resources a fair tradeoff could be trading
result size with less memory and cpu consumption. Expose a variable
that overrides the default (9).

Note: xz is often used to gain maximum possible compression, so
avoid lowering this if possible.

PR-URL: https://github.com/iojs/io.js/pull/1428


Reviewed-By: Rod Vagg <rod@vagg.org>