Use cmake to build runner dependencies. (#4238)

Summary:
Pull Request resolved: #4238

Building some of the llama runner dependencies like RE2 and Abseil is non-trivial via Xcode, so let's make Xcode leverage cmake to build those.
The downside is the customers will need to install cmake on their Macs, but that's a tolerable con.

Reviewed By: kirklandsign

Differential Revision: D59672358

fbshipit-source-id: 3ce630926143293e495250e9f498ee78993397e0

Author: shoumikhin