./miniruby.exe is not yet available, but there is an attempt to use it. I think the solution is to either use base ruby here, or to delay the production of the files derived from Unicode data files to a later stage.
I do not get this error from a preexisting checkout, obviously because ./miniruby already exists. But this problem should also affect continuous integration, unless it is for some reason cygwin-specific.
I'm attaching a configuration summary and the full make output.
I apologize for having pressed the submit button erroneously when I barely started writing this bug report.