skeleton for multitarget

[smparkes]

This is a follow-on to the configuration PR. It shows the basic idea of using a macro shim to create multiple targets based on the configuration.

It's not correct yet: it doesn't handle the different versions correctly. What it does do is include documentation on the approach to give folks a chance to comment on.

I'll work on flushing out the implementation but would welcome thoughts on the approach.

[smparkes]

Works for the current example builds. It should be creating and running parallel subgraphs for each version.
The code could use cleanup but isn't horrible. I'll do more cleanup passes as things progress.
For now I want to move on to actually creating and using correct toolchains (everything right now says it's using alternate toolchains but they're all configured with the default toolchain params.)

[smparkes]

Moving this to a PR on upstream.