The header X11/Xw32defs.h seems to be missing when you download the headers via XFree-86-headers in Cygwin's setup.exe. Thanks, -Ken c:\Cygwin\usr\X11R6\include\X11\Xos.h(142) : fatal error C1083: Cannot open include file: 'X11/Xw32defs.h': No such file