build: use BUILDTYPE when building V8 in Makefile

Without this it would always compile Release and Debug builds.

Ref: https://github.com/nodejs/node/issues/7477
PR-URL: https://github.com/nodejs/node/pull/7482


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>