FindCython doesn't find local cython.

Detect the cython executable in the same path as the python
executable used in a local installation.

Reported by Paul Romano.
Update FindCython.cmake from upstream (thewtex/cython-cmake-example)
Merge pull request #123 from thewtex/CMakeCythonExePath

FindCython doesn't find local cython.
Merge remote-tracking branch 'upstream/staging' into mesh

This is an update to staging. Any future PRs will be rebased first to avoid this.