route-rnd fails to cross build from source, because it builds for the
build architecture. It is treated as an autoconf project by debhelper,
because there is a configure script, but that configure script ignores
all arguments and environment variables and does not configure anything.
The crucial piece to making this work is running dh_auto_build as a
makefile buildsystem where debhelper passes cross tools. In the attached
patch, I opted for including some related improvements and hope you
agree with them. Let me know if you prefer a minimal patch.

Helmut