I almost thought about removing the checks from the source and compiling by hand but,
fortunately I have found a way around this :

Use this information at your own risk, I can only say it worked for me so far, in my small 5 user home installation ...
- set the data destination for an external account manually :
Code:
zmprov mds yourZimbraEmail@YourServer.Suffix zimbraDataSourceName zimbraDataSourceFolderId '2'
This assumes that your Inbox Folder ID is 2 (which it is in all my accounts, but I cannot tell for certain if that is always the case ...)
You can get the zimbraDataSourceName with :
Code:
zmprov gds yourZimbraEmail@YourServer.Suffix
You can also use this to check if it's set correctly now.
This works very well for me (Zimbra OSS 6.0.8) , all messages from 4 different accounts are dropped to Inbox.
Beware - if you change something in the WebInterface in this particular external account - you probably have to repeat above instructions ...