build: fixes for Windows

* work around a linker/cgo bug
* do not run deps.bash on Windows unless we need it
  (cuts a full minute off the build time)
* add windows to the list of cgo-enabled targets

The gopack problem is issue 2601.

R=golang-dev, r, bradfitz
CC=golang-dev
https://golang.org/cl/5504062
3 files changed