The configure script that comes with qmail-scanner seems to incorrectly detect a properly patched qmailqueue when in fact the patch has not been applied.
Don't worry though, qmail is VERY easy to patch and re-compile. Just follow these steps:
1. Go to your qmail source directory
2. Fetch "http://www.qmail.org/qmailqueue-patch"
3. Delete everything ABOVE the line "diff -u qmail-1.03-orig/Makefile qmail-1.03/Makefile" (cut away the email message, leaving only the patch)
4. Apply the patch with "patch < qmailqueue-patch"
5. Stop qmail with "service qmail stop"
6. Re-compile and re-install qmail with "make setup check"
7. Re-start qmail with "service qmail start"
qmail-scanner should now work properly.
I told you it was easy :-)
(Thanks to Ulva for this solution)