I mistakenly merged in an intermediate state of the issue326 branch 
instead of your default at ba9817cdaa2ad6a76591e5a53043653ec6fb6444

My GUI's graph display didn't make it clear that this was upstream branch 
not upstream default.

OK, I am clearing this bug, since it was mercurial user error.  And I am 
banishing murky from my life!  (MacHg made it clear what I did wrong, for those 
interested in a Mac hg client...)

No, I am afraid I don't use Windows so can't really help with this.  As I said, 
the merge with your branch was a problem.  I have a colleague who uses cygwin 
all the time, but he does not use FD, so could only perform relatively simple 
tests.  LMK if this is really blocking you, and I will ask around, but we are 
primarily a Mac + Linux shop.

I found the three lines now; they were not in the default branch, but in the
Cygwin integration branch (issue326). They were the remnants of some tests that
I left in by mistake.  I've removed them now; if you merge from the current
version of issue326, things should work again. I'll close this, but of course
feel free to reopen if the problem persists.

Incidentally, regarding Cygwin integration (issue326), we're a bit stuck there,
to a large degree because I don't have a Windows machine to test anything. Since
it looks like you tried to merge that code: do you have Cygwin access? 

Does the issue326 branch work for you? If yes, it'd be great if you could
comment in the discussion there. If not, it'd be very useful, too.

It is quite possible that I have gotten my copy of the hg repository into a bad 
configuration: the UI that I use does not display branch information, and 
perhaps when I pulled I inadvertently merged together two upstream branches.

hg branch is "default"
hg identify is "627a78439422+ tip"

Let me see what else I can discover, now that it appears this is my bug.

I can't find this. Can you tell me the output of

$ hg branch
$ hg identify

where you see this?

Pulled update on the 22nd and find this in search/utilities.h:

#if OPERATING_SYSTEM == OSX
#error WHAT?
#endif

Pulled again, just now, and it's still there.  AFAICT this was introduced in 
commit 2944 at the same time as code to detect CYGWIN.

Is this debugging code that accidentally was left in?  I deleted those lines 
from utilities.h and the compile on Mac OS X seems to run to completion just 
fine.