Remove generated documentation, python and mruby build artifacts. Note that this does not work for Ninja, Makefile works fine though.
FindCython.cmake was taken from https://github.com/thewtex/cython-cmake-example. The UseCython module works, but since it is lacking an installation target setup.py will be used instead.
Might not be the best approach, maybe use thewtex/cython-cmake-example?