I will add some comments for this line later.

This is the closest I found to official documentation: http://www.cmake.org/pipermail/cmake-commits/2010-December/009738.html

When you build in Cygwin, you get a warning, of which the relevant part is:

(2) If you are developing this project, add the line
  set(CMAKE_LEGACY_CYGWIN_WIN32 0) # Remove when CMake >= 2.8.4 is required
at the top of your top-level CMakeLists.txt file or set the minimum required version of CMake to 2.8.4 or higher.  Then teach your project to build on Cygwin without WIN32.

YouCanComplete will build on Cygwin without WIN32, so I added the "set(CMAKE_LEGACY_CYGWIN_WIN32 0)" line as instructed.