libgpiod fails to cross build from source, because the Python extension
is built for the build architecture. There is a tricky environment
variable one has to set and when using the pybuild debhelper
buildsystem, this is handled automatically, but not when building a
Python extension inside autoconf. Thus it needs to be set explicitly.
Then it fails for non-default Python versions as only libpython3-dev is
included in build dependencies and libpython3-all-dev is missing. The
<!cross> annotation on help2man is a lie, because debian/*.install fails
when skipping help2man. I have no good answer to help2man unfortunately
and that's what makes it continue failing for now. The attached patch
fixes the other mentioned issues.

Helmut