Make all #includes relative to the including file
This allows <Lib$Dir>.tboxlibint to be removed from our command line include path. Henceforth, clients of this library also won't need to put it (or C:tboxlibint) in their command line include paths either. It also means that when building the library, only local copies of header files are used (as opposed to a mixture of local and exported copies, as previously).