The first step towards `C5T_DEPS` and more repos under `C5T`.

[dkorolev]

Learning cmake step by step.

This "development process" is slow because GitHub actions work on the stable branch of upstream.

But this seems to work on my local test.

And: it seems to work both for a header-only repo and for a non-header-only, library, repo!

[dkorolev]

@mzhurovich I'll merge for now to proceed. Post-review pls!

Here is a short & sweet example that works now: https://github.com/dkorolev/example_use_current_with_cmake/commit/d55a7dc23fefd85e1cb1a7e63ca00bc4b3c2247f

[dkorolev]

@mzhurovich -- and here's the cleanest possible example:

• repo
• adding a dep
• all it takes to run
• gh action