Only take md5sum if there is no pnm output file.
Only take md5sum if there is no pnm output file.

darcs-hash:20100116215052-f546f-fc0e21aa5841e83e11af04e8ded3874e32fda54c.gz

nqv/mupdf,nqv/mupdf,nqv/mupdf,nqv/mupdf,nqv/mupdf,nqv/mupdf