[setup] allow easy override of the ocaml version
[setup] allow easy override of the ocaml version

Reviewed By: mbouaziz

Differential Revision: D5397957

fbshipit-source-id: b5186f4

blzq/infer,jvillard/infer,jvillard/infer,facebook/infer,blzq/infer,facebook/infer,ropas/infer,jvillard/infer,ropas/infer,ropas/infer,jvillard/infer,facebook/infer,jvillard/infer,blzq/infer,facebook/infer,blzq/infer,jvillard/infer,facebook/infer,facebook/infer,ropas/infer,facebook/infer,ropas/infer,ropas/infer,blzq/infer,jvillard/infer,blzq/infer