sane-backends implemented a new structure with version 1.0.20 in 2009, where SANE extensions (like HPLIP) are supposed to place their files in /etc/sane.d/dll.d and NOT modify /etc/sane.d/dll.conf
This was addressed in Debian/Ubuntu http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=512596 but apparently was never applied upstream -- probably not even offered, from what I can tell. The Debian patch, however, looks only in the file dll.d/hplip and not in all dll.d files as it's supposed to.
The problem arises when sane-backends is updated, stomping on dll.conf and removing "hpaio" from the list.
I have attached patches that address this issue. I note that the documentation and check.py appear to assume the files will be in /etc rather than in ${DESTDIR}/etc, but I did not try to resolve that discrepancy. The Makefiles handle this correctly. I also note that hplip.list.in appears to work only with /etc rather than ${DESTDIR}/etc, but again I did not try to resolve that discrepancy; for all I know it's correctly modified somewhere in the config process.
