Unbreak the Windows build

The v8 team apparently decided that all build products should go
into ./build/«type», and updated their common.gypi file to do so.
Unfortunately v8's common.gypi is only used for some targets. All
the other targets would still look in the old place to find their
their dependencies, which effectively broke the build.

In the long run it would be good for node to send all build
output to ./build too, on all platforms.