using cling under cygwin

build instruction:
- https://solarianprogrammer.com/2012/09/02/building-cling-cpp-interpreter-windows/
- https://root.cern.ch/drupal/content/cling-build-instructions

fix some issues:
- http://stackoverflow.com/a/21820642/661935
- http://stackoverflow.com/questions/6525245/getting-clang-to-work-on-windows

test:

now, you can do more magic things with cling.