In PellesC has changed nothing.
In the window headers something has changed, but you are using 64bits version I suppose, and in this case an handler, returned by GetStockObject, has not the same dimension of a WPARAM (a void pointer when WPARAM is a 64 bits integer).
Simply cast the function to a WPARAM and you'll solve the problem.
SendMessage(hwnd, WM_SETFONT, (WPARAM)GetStockObject(ANSI_FIXED_FONT), 0);
Anyway it is always a good habit to cast the parameters of a SendMessage to the correct type.