cmark cannot be cross built from source, because it requests the host
architecture Python interpreter via Build-Depends and that cannot be
installed. Looking closer, it wants to run Python during build, so we
should annotate the dependency :native. Looking even closer, it runs it
as part of its test suite, so we can also annotate it <!nocheck>.
Reproducible builds confirm that a nocheck build does not require Python
while still yielding the exact same packages. I'm attaching a patch for
your convenience.

Helmut