It's only needed for a hardcoded int, which we can define locally.
Note from Backporter: Core's description of this is indeed somewhat confusing. nScriptCheckThreads is removed in the next commit.
https://github.com/bitcoin/bitcoin/pull/17342/commits/d9957623b48a7c3eff0ac750d1245fabfb1843a2
Partial backport of Core PR17342