Unbreak build when no system ocaml installed
Unbreak build when no system ocaml installed

Summary:
If ocaml is not installed (or if homebrew has sandboxed it out), `opam
init` fails unless passed `--compiler`.

Reviewed By: sblackshear, jvillard

Differential Revision: D3927532

fbshipit-source-id: d2fbec9

jvillard/infer,jsachs/infer,kageiit/infer,xennygrimmato/infer,jvillard/infer,kageiit/infer,ryoon/infer,ropas/infer,xennygrimmato/infer,facebook/infer,facebook/infer,ryoon/infer,xennygrimmato/infer,blzq/infer,blzq/infer,ropas/infer,facebook/infer,jsachs/infer,jvillard/infer,xennygrimmato/infer,jvillard/infer,blzq/infer,kageiit/infer,facebook/infer,jsachs/infer,ryoon/infer,jsachs/infer,ropas/infer,facebook/infer,jsachs/infer,ryoon/infer,kageiit/infer,jvillard/infer,facebook/infer,ropas/infer,blzq/infer,jsachs/infer,ropas/infer,xennygrimmato/infer,ryoon/infer,kageiit/infer,xennygrimmato/infer,jvillard/infer,ryoon/infer,jvillard/infer,ropas/infer,kageiit/infer,xennygrimmato/infer,blzq/infer,blzq/infer,kageiit/infer,facebook/infer