Summary:
VERSION_GREATER 3.18 also matches 3.18.2, which is wrong as the target argument was only added in 3.19.
After noticing that folly requires an ancient CMake version I converted over from VERSION_GREATER_EQUAL and introduced the comparison error.

Issue reported [here](https://github.com/facebook/folly/pull/1433#discussion_r484278801).

Pull Request resolved: https://github.com/facebook/folly/pull/1439

Reviewed By: yfeldblum

Differential Revision: D23564812

Pulled By: Orvid

fbshipit-source-id: 19881876147ec73ad70610cf97c408740f998340