fix boost build on single-core machines
fix boost build on single-core machines

Summary:
On a single-core Ubuntu VM, getdeps attempts to pass -j0 to Boost Jam,
which fails.

Reviewed By: fanzeyi

Differential Revision: D33675531

fbshipit-source-id: 629ee41448941213aadec10658583d8f984b13c2

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst