FetchContent: Add wait time between git clone attempts

The current approach to git clone by FetchContent/ExternalProject is to try 3 times in a row to git clone a given repository. It would be nice to add a configurable wait time between retries, to be more resilient to transient network issues. I guess I could wrap the git command itself but that seems a bit complicated.

Edited Feb 13, 2023 by Brad King

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