How to write cloud-agnostic Terraform code

Terraform is a beautiful tool we all love; it is multi-cloud, meaning you can write code to provision resources from all major and many minor cloud providers. Sadly the code you write is vendor specific as resource descriptions are not portable between providers. What if your boss or customer wants a cloud-agnostic solution? Do you need an external tool or Terraform language is rich enough to allow cloud-agnostic code? I stand that 0.12 is so powerful that you can create provider-agnostic modules and write code that works the same in GCP, AWS and Azure.