[LinuxCNC/linuxcnc PR#75] Jepler/2.7/external rs274

IRC user ickalB reported that his out-of-tree use of librs274 stopped working. I investigated, and found that the report is accurate—ever since I introduced the InterpBase class, it has not been possible to create an Interp object using only public headers without resorting to gross hacks.

This fixes the situation, and also adds a test so that we learn about the failure sooner than three years after the fact.

I think this is a candidate for 2.7, but I’d be happy for it to go into any branch.