bug#19686: Bad source checksum in netpbm

[Andreas Enge]

This is a tricky error. I still had the svn checkout of the previous version
on my disk:
   /gnu/store/4a6ck86ap94m397vmycxy98bmhws7h6a-svn-checkout
So the recipe worked for me, since the hash and the file name corresponded to
the package recipe. However, instead of the new, I actually compiled the old
version and stored it under the new name in the store. And also hydra compiled
it without complaining.

I have corrected the problem for netpbm. However, I think we should move to
more descriptive directory names, where "svn-checkout" should be replaced
by (string-append name "-" version) as usual.

Andreas