I'd prefer the following behavior: Let the user specify whether to use libndofdev or not. (Of course we'd have to agree on a default if nothing was specified). Then ...

... if libndofdev use has been enabled and libndofdev can be found,
  --> build with libndofdev support, thus with the joystick/spacenavigator features.

... if libndofdev use has been enabled but libndofdev can not be found,
  --> error out. (With some meaningful message)

... if libndofdev use has been disabled,
  --> don't build with libndofdev support, even if libndofdev is present.

Rationale:
If I *want* spacenavigator or joystick support, but libndofdev cannot be found (e.g. because I forgot to install it or because it was installed to a location that isn't searched), I'll get a heads-up at build time rather than having to figure out why stuff doesn't behave the way I want at runtime.

Note:
This would be analogous to how we currently handle FMOD, even although disabling FMOD -- other than disabling libndofdev -- doesn't result in unavailable viewer features.