0xffff fails to cross build from source, because it builds mangen with
the host architecture compiler and thus fails running it. The upstream
build system gets a lot right. It differentiates between CC and HOST_CC
and uses the latter for mangen. Unfortunately, dpkg/debhelper call that
thing CC_FOR_BUILD, so we need some renaming here. Additionally, the
upstream build system does not differentiate between flags of the
compilers, but on Debian the flags have become architecture-specific. So
we additionally need to set e.g. HOST_CFLAGS (upstream) as
CFLAGS_FOR_BUILD (Debian). The difference in nomenclature is
unfortunate, but not something to be fixed here. I'm attaching a patch
for your convenience. Can you try upstreaming the cross.patch?

Helmut