I apologize if this response took longer than you expected. I still have to periodically dig out your e-mails from my Spam folder (I now at least know to check there).
Anyway, yes, I know this is a problem and I've been meaning to add an error whenever the ICU headers are not present. The code is still very much intertwined with calls to ICU, so I better get this done sooner rather than later.
That said, do you have the ICU headers installed? If so, where? I'm wondering because configures gives out an error when "icu-config" is not present, and, generally speaking, if icu-config is present so are the headers. The configure script is likely to contain bugs since I've been learning Autoconf as I go.
By the way, thanks for periodically building corebase. Since I get so few bug reports, I never if anyone is still doing so. I'm working towards getting a second release out before the end of the year... Assuming I can squash all the known bugs before then.