Explain "#ifdef CURVE25519_ENABLED" better

I find the CURVE25519_ENABLED ifdefs really confusing. It would seem to mean "can we do ntor handshakes", but if it does, why is there a bunch of ntor code not covered by ifdefs? When should I check ifdef when doing something ntor-related, and when shouldn't I? Are there things that aren't ntor-related where it matters?

I was about to send Nick mail asking, but then I realized that other / future Tor developers should want to know the answer too.

Thanks!