cygwin cmake looks for .so instead of .dll

This should be relatively simple to fix. I just need to figure out how to determine if we're in cygwin in cmake. I will also silence the annoying warning we get currently in cmake/ccmake on cygwin. This shouldn't be too tough. By the way this error is happening at 100% of the build process, so crossing my fingers that this is all it takes to get a cuda/itk-free build on windows up and running.