Just re-set up a Windows dev box. It looks like at least the following Cygwin packages weren't installed that needed to be to run various WebKit scripts: curl diffutils patch perl ...and their dependencies. (Something could have gone wrong on my end; I had to install Cygwin twice to get the relevant files in /etc/setup to do a rebaseall.)
I did a clean installation a couple of weeks ago, and I got the aforementioned packages with cygwin-downloader, no manual tweaking was necessary.
OK, I'm willing to believe that something went wrong on my end somehow.