From ebuild sane-backends-1.0.20 onwards, each SANE backend is selected to be compiled/installed by a USE flag sane_backend_NAME. The list of accepted backend names is set by IUSE_SANE_BACKENDS in the ebuild file. Unfortunately this list does not include "pnm", so it is impossible to build this backend even if it is wanted. (It is useful because it is the only "testing" backend that includes gamma table support). There have been security concerns with this backend, so it is reasonable to disable it by default. But it should be possible to specify that it should be built and installed if it really is required.
Good idea. It is slightly more work than just adding it to the list since it is handled separately from other backends because of the security concerns, but I've added it now.
