How Do I Get The Wifi Signal Strength Using C Or C++ Code?

Entirely depends on the application.
Wifi modules (in mobile devices or otherwise) are unique to the module. iOs, Android etc have standardized APIs (Application / Program Interface) for getting that info. PC's have a choice of hardware so each device (or at least manufacturer) would have their own API. In many cases you'll have trouble getting hold of the API for PC, but there's plenty on Google on how to access smart phone wifi apis