[easy] fix `make clean` when ocamlbuild isn't installed
[easy] fix `make clean` when ocamlbuild isn't installed

Reviewed By: vrama628

Differential Revision: D27232035

fbshipit-source-id: 1684d06abf99a53687da2a18d130cc50c45a8ca9

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