The colons there don't represent C++. That's just a way of referring to a windows API function that exists in a specific DLL (in this case "user32"). Because the functions used here do not exist in older versions of Windows, the linked code dynamically loads user32.dll and tries to get the address of those functions so they can be called. That's why you need to know which Windows DLL they exist in.
Was going to point out would have to be an embedded program that can be booted into without need of an OS; but then program would still rely on various external api/abi interfaces aka bios, drivers & protocols (usb, scsi, pci, ansi, etc). Given today's hardware, think there's not really a realizstic thing as a realistic reall world 'pure C' program -- way to much knowledge / effort even if exclude NDA's. VM environment for 'hardware' as software still has abi interface, so not 'pure' C at source code level.
