Make the package build-depend, depend, build, and work with only
fftw3-dev. There is no need to have fftw-dev installed in the system,
but this needs the 50-either-fftw-or-fftw3.patch patch, which applies
to configure.in. This patch must be sent to the upstream authors.
Unfortunately, dpkg-shlibdeps does not detect the needed dependency on
fftw3, because there is no executable or shared library that depends
on it. This dependency was added by hand to debian.control.