updates in cmake build

Hello,

I've done following updates in cmake build:
- added missed header keyvalq_struct.h to install
- compatibility headers v1.x will be installed to CMAKE_INSTALL_PREFIX/include
- added check of Python version (event_rpcgen.py will not work with Python >= 3.0.0)
- updated LibeventConfig.cmake.in with correct install path
- cosmetics: removed trailing spaces from CMakeLists.txt

Could you please review and merge?
Thank you.