> 
> If it really exists in /usr/include, then it means your compiler does not 
> look there.
> 
> Maybe you are not using the ruby interpreter built by cygwin. You have to. 
> You cannot use cygwin to build ruby extensions for a ruby interpreter 
> built with another compiler.
> 

I have never installed with Win32 Native Ruby-- only Cygwin

> Or you have multiple installations of cygwin.
> 
> Try executing extconf.rb as
> 
> /usr/local/bin/ruby extconf.rb
> 
> from your bash prompt. 
> 

Ruby was installed in /usr/bin rather than /usr/local/bin.  This 
installation was done using the cygwin setup.exe on June 8 so I am 
fairly current.  I have a sym link at /usr/local/bin/ruby because this 
is where it is on my other systems...

culley