BLD: try harder to locate Cython executable, it's not always on the path.

Closes gh-2397.
BLD: change Cythonizing in case of no "cython" exe to suggestion @pv on gh-2449
MAINT: ensure exit code from cython
Merge pull request #2449 from rgommers/issue-2397-cythonize

BLD: try harder to locate Cython executable, it's not always on the path