You can use dependencies. You just need to cache them internally, review them meticulously, and never upgrade them without going through the review process again. It's a huge pain but some companies do it.
Note that the distributions used by package managers such npm, pip, or cargo do not do this. So be wary of the "all old is bad and needs to be rewritten and C does not even have a proper package manager" crowd. (memory safety is a good thing though)