[Mimedefang] Image validator/OCR SA plugin

[Cormack, Ken]

So far in my tests, this OCR plugin looks like it's working ok.  I rounded
up the needed prereqs (that was a bit of a chore, but everything compiled
cleanly), and changed the package definition as indicated in Martin's post
(be sure to run "spamassassin -D --lint").  So far I've seen several hits
for the ocr SUSPECT_GIF rule, with no detectable problems.

Ken