Summary:
This took a lot of cmake fuckery. I am guessing it will break the current travis-ci.org based build, since it no longer downloads gflags.

As I put this diff up, mvfst (a dependency of quic (a dependency of proxygen)) is not building right now due to another diff - a fix is in progress but isn't landed yet. I applied the fix locally for the test plan, but didn't include it in the diff.

Reviewed By: snarkmaster

Differential Revision: D25651851

fbshipit-source-id: b4f15ac7fa5293123710fe367c20d355c0970c42