I was able to work around this problem by: 1) making a symlink /usr/lib/cups/filter/pstoraster to /usr/lib/cups/filter/gstoraster.
2) Copying hpgltops from another distro (I have Crux which uses an earlier version of cups). No.2 is frankly a bodge but no.1 makes sense because gstoraster provides the same service as pstoraster.
