One approach I've seen in many NI-distributed VIs is a ring type definition. Rings are used because the values in them do not need to be sequential; the downside is that unlike an enumeration, a the items in a list aren't part of the data type, so they don't update when new items are added to the type definition. If the DLL header is stable and unlikely to change then this may be acceptable.
If you need flexibility to change the #define values, then you could create an enumerated type definition and a dedicated VI that uses that enum as an index into an array of output values. It's very slightly more work but much easier to change if the header changes.
