Weird, I am having the same problem just now and looked here for an answer.
I’ve built OpenWRT for all sorts of devices, this is the first time I had a problem.
It looks like the latest trunk and 8.09.1 do not work with the metarouter kconfig.
So I svn’d version 17511, which looks to be the latest rev that nik247 built in this post http://forum.mikrotik.com/t/metarouter-images/28791/1 and it is compiling as I write this.
I’ll let you know if it works.
EDIT: error was a kconfig.pl parse error on line 699. I deleted the files so I cannot reproduce it.