Add support to build kubectl with cgo

[bks7]

Modify formula to support build dynamic binary with CGO_ENABLED=1.
Building kubectl without cgo breaks dns name resolution since it
uses /etc/resolv.conf for getting resolvers while nothing in macos
should use it for name resolution.
More information about this issue can be obtained here -
kubernetes/release#469

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[ilovezfs]

Thanks for the PR, @bks7. This is something you'll need to get sorted out with upstream, though, as we don't want to add a temporary option in the formula just to work around an issue CC @cblecker

[bitglue]

@ilovezfs Upstream says it needs to get sorted out downstream. kubernetes/release#469 (comment)

kubernetes/release is designed to run in a docker container within a Kubernetes cluster, and since installing Xcode in such an environment isn't possible, it disables cgo. I don't see a reason for Brew to be limited by those constraints. It would be nice if at least one of the documented install methods had working DNS. Will you reconsider?