Fixes .gyp scaffolding for building on Windows.

Patch from Refael Ackermann <refack@gmail.com>.
https://github.com/nodejs/node/pull/16848/commits/5528afd073efe3fe4f5019887f30134d649abc16

Bug=v8:7061

Change-Id: I4faaf3f488b6725942746d74838ef7ce73b1e8d0
Reviewed-on: https://chromium-review.googlesource.com/761477Reviewed-by: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49280}