superlu fails to cross build from source, because its gfortran
dependency is not cross-satisfiable. In general, bare toolchain
dependencies are not compatible with cross compilation, because they do
not indicate the architecture the toolchain is being used for. In this
case, gfortran-for-host should be used.

However, when using gfortran-for-host, the bare "gfortran" is not
available for native builds. The triplet-prefixed
"${DEB_HOST_GNU_TYPE}-gfortran" should be used instead. Arguably,
explicit is better than implicit and this variant always works.

I'm attaching a patch that implements this and lets dpkg's buildtools.mk
correctly initialize FC. The patch is meant for forky, not trixie.

Helmut