Thanks for both of your replies. mratsim: I don't think that approach will give me quite what I want as my import dependencies go the other way. Rather than defining the flag in a file and deciding which file to import from there, I need to import a particular file from each of two places and have it do slightly different things in each case.
mashingan: That is what I need. The yourfile.nim.cfg file is exactly what I need to define this for the compiler. Thanks again!
