Apply OCaml deprecation fix from HHVM-3.3 branch

Hi @jwatzman,

I've sent #3835 to fix the deprecation for OCaml4, but I guess you don't want to break OCaml3 users.

This pull request is applying OCaml deprecation fix from HHVM-3.3 branch to let HHVM-3.2 works for OCaml.



Fix external build?

Summary: One external report that this unused variable was causing an
error in the build. Our internal build went fine, so this might be a
change in default warnings between 3.12 and 4.01? In any event, easy
enough to fix, and also remove warn-error in the external build for now.

Reviewed By: @gabelevi

Differential Revision: D1512305

Conflicts:
	hphp/hack/src/typing/typing_decl.ml