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
Merge pull request #3868 from c9s/HHVM-3.2

Apply OCaml deprecation fix from HHVM-3.3 branch