Run the following from the location of the terraform git clone:
docker run --rm -v $(pwd):/go/src/github.com/hashicorp/terraform -w /go/src/github.com/hashicorp/terraform -e XC_OS=darwin -e XC_ARCH=amd64 golang:latest bash -c "apt-get update && apt-get install -y zip && make bin"
Resulting binary is in pkg/darwin_amd64.