run ocamlbuild in parallel
run ocamlbuild in parallel

Reviewed By: samwgoldman

Differential Revision: D21639929

fbshipit-source-id: 7e0f066c0fff647b1caee973aa3cd9b702d78309

facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow