Visual Studio 2015: C++ decorations despite extern "C"

I'm trying to use an SDK from that I have a H and OBJ file. The H file declares all functions as extern "C", yet while linking, I get "Can't resolve external symbol _ibclr@4". The function is called ibclr and takes a 4B-parameter, so it looks like that's just the C++ decoration. But how is extern "C" not working?