MAINT: always regenerate Cython files, except for when building from sdist.

The code that's replaced here would not do the right thing when building from a
checkout of a release tag in a git repo.

See https://github.com/scipy/scipy/commit/c0124923b for discussion.
Merge pull request #493 from rgommers/when-cython-regenerate

MAINT: always regenerate Cython files, except for when building from sdist.