Tracking Issue: Abseil CMake Support

[JonathanDCohen]

There's been a lot of scattered discussion with respect to Abseil's CMake support. We are now prepared to offer a sequence of milestones for Abseil's CMake support. This tracking issue is meant to be a place of discussion on this sequence and a message board for when we are going to make potentially-breaking changes to our CMake files. Individual milestones will be tracked in the Abseil CMake Support Project as separate issues, for more detailed discussion.

[asoffer]

All issues have been completed.