[Rd] Watch out for the latest Cygwin upgrade

[Duncan Murdoch]

I just updated my copy of Cygwin to the latest version, and now Windows 
builds are failing on that machine.  The only parts of the R toolset I 
changed were the Cygwin dlls.  I haven't tracked down exactly what the 
problem is, and probably won't be able to do so for a few days.

So if you're a Windows user thinking about a Cygwin upgrade, be prepared 
for problems...

Duncan Murdoch