liblopsub fails to cross build from source, because it builds for the
build architecture. The easiest way of fixing this usually is using
dh_auto_build, but this package appears to avoid debhelper where
possible. Hence, I propose using dpkg's buildtools.mk instead. Then, the
upstream build system needs reworking to support cross compilation. It
builds two intermediate binaries run during build. For lopsubgen-stage1,
we may just build it for the build architecture as it is not installed.
lopsubgen however is both run and installed. I suggest building it twice
(but only during cross compilation) and am attaching a patch to this
end. Please propose this patch to upstream as it will enable cross
building liblopsub on other distributions such as Yocto or PtxDist.

Helmut