That is a really bad idea. The windows API libraries have been tested extensively, and if you start changing them then you are likely to create far more problems than you will solve. The include file clearly shows you the format of your DLGPROC function, so it is up to you to create it to match that function definition; it returns an INT_PTR not a BOOL. You can also add a cast in your DialogBox() call to help the compiler along.
One of these days I'm going to think of a really clever signature.