fftw3 fails to cross build from source, because its gfortran dependency
is not cross-satisfiable. Generally bare toolchain names cannot be used
for cross compilation, because they do not indicate whether they're
meant for the build or host architecture. In this case,
gfortran-for-host is what is needed.

However, gfortran-for-host cannot provide the bare "gfortran" name.
"${DEB_HOST_GNU_TYPE}-gfortran" must be used instead. We looked for
alternatives and they all regressed some use case. That said using
triplet-prefixed tools is explicit and works reliably even when building
natively.

I'm attaching a patch for your convenience, but I think applying it
should be deferred to forky.

Helmut