`make demo` hard to run

[osfameron]

make demo doesn't work on OSX as there's no ps2jpg executable.

I can't find where to download that from, but no problems, the README says to just edit the Makefile. OK! I've tried a likely looking invocation with sips but it produces a zero-byte file.

I'd like to debug it, but the Makefile insists on deleting the .ps output ("Removing intermediate files"...) This seems unnecessarily aggressive (after all, the .ps output is surely of interest in the first place?)

I don't know enough Makefile to debug (I've had a play with .PRECIOUS and so on, but I Don't Really Know What I'm Doing) so giving up for now, but thought I'd raise ticket for the record.