I already tried your script on a machine with Cygwin installed but
without iconv and saw the same complaints from configure in your build
script. There are other configure flags for this package which could
help if you want to experiment with them (assuming you haven't already).
But also doing what the configure script recommends solves the problem.
If you're content to do that, then you have a solution. If you're not
(as I expect you aren't) and the configure flags don't help, then I'd
recommend talking to the package authors about the bump in the
configure process you're noticing.