The WICED and libraries are writen in C. The "application_start" is the entry point for user applications to start executing on an main thread, you may think like it is the main() for the application. The application_start() is called by the main thread to initiate user application.
Are you trying to write your application in C and save it in .cpp extension file names?
I have a set of libraries written in .cpp that I wish to add/use in my application.
Update: I was able to get away with that error by adding extern "C" in my main application file.