readsb fails to cross build from source, because it builds for the build
architecture. debian/rules does not pass any cross tools to the make
invocation. The easiest way of doing so is using dh_auto_build. Even
with that change, the upstream Makefile still hard codes the build
architecture pkg-config. The attached patch both uses dh_auto_build and
makes pkg-config substitutable. Please consider applying it.

Helmut