key signature for modal music

I hesitate to ask this question, for I don't want to start a debate. My hope is that there is a standard answer here, but if responses disagree, then I'll assume there isn't and I'll just leave it at that rather than try to resolve things.

When writing standard music notation for a modal melody, what do you indicate for the key signature?

For a simple dulcimer-friendly example, if we were to write out the music for Old Joe Clark in the key of D, should we indicate one sharp (F#) or two sharps (F# and C#) in the key signature?  In the first case, the key signature would accurately indicate the scale used, since the song is in the mixolydian mode and uses the C natural, but it would wrongly indicate the song was in the key of G.  In the second case, the two sharps would accurately indicate the key (D) but would inaccurately indicate the scale being used and would necessitate C natural indications in the notation.

In my mind there is no satisfactory answer.  But there might be a standard answer.  How is this issue "normally" handled?