Various fixes for MacOS Mojave 10.14 (#3)
Various fixes for MacOS Mojave 10.14 (#3)

Summary:
I cloned this repo for the first time today and ran into a few problems building on MacOS Mojave 10.14.

Here's a small number of standalone fixes to the `build_helper.sh` script.
Pull Request resolved: https://github.com/facebookincubator/mvfst/pull/3

Reviewed By: lnicco

Differential Revision: D15212179

Pulled By: udippant

fbshipit-source-id: 86b5619954851654cc756d0b238c89896d7b42d1

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