Okay, now it is at least running well enough to start the test script.  The
trick was to change my Windows "%path%" environment variable to put
c:\cygwin\usr\bin earlier than c:\WINDOWS\System32, so that the "sort.exe" that
gets called by GNU make when it wants to launch a test run is unix sort.exe and
not Windows sort.exe.

Now there are a bunch of test failures, some of which no doubt need to be fixed
"on my end", such as further configuration of my %path%, others of which no
doubt need to be fixed "on the darcs end", such as by changing darcs to work
better on Windows.

That's my assumption anyway.  We'll see...