You'll need to have libfftw3 installed. On my Linux machines, I needed to configure that with --enable-shared. The results we get are slightly different from those of Mark Newman's stand-alone cart program. For example, when using uspop.tab data, we get 1.002184 and 2.004368 for x = 1 and 2 (using a grid 0, 1, 2, ...) whereas Mark's code yields 1.00214 and 2.00428. This seems to be a feature of using a different grid (1:1024 rather than 0:1024 and similarly for y). See tests/check.R.