Make dependency on lz4 optional for OSS build.

Summary: Flow pulls in Hack, but it shouldn't have to require lz4.

Also copied over@jwatzman's fix for the C option handling in pre-4.00
ocamlbuild (c.f. D1692564)

Reviewed By: @jwatzman

Differential Revision: D1693464

Signature: t1:1693464:1416520127:5508177943e51420cce3606a64e340413f54e06e