Summary:
The *string* `"false"`, which was being returned by default, will be interpreted as truthy. So we were never getting the `-j` arg on the `make` command.
Should be a nice little speed boost for some builds.
Reviewed By: chadaustin
Differential Revision: D78819581
fbshipit-source-id: a1b57f17185d189df25e913e06bcf71f8f540053