gauche fails to cross build from source. The immediate failure is due to
not finding libatomic-ops. The relevant configure check is pessimistic
about atomic compiler intrinsics and opts for libatomic-ops as a
fallback. The gcc provided by Debian does provide suitable intrinsics,
so telling configure about that choice explicitly makes it move forward.

Later, the build system expect a native gauche to be installed to run
scheme code during build. This is specific to cross building and we need
to ensure that gauche as well as the dbm module are available. This is
expressed as a recursive build dependency annotated with the <cross>
profile. Since it is a positive profile, it does not affect native
builds.

Please consider applying the attached patch as it makes gauche cross
buildable.

Helmut