Don't build Python modules unnecessarily before symlinking to site-packages
Only ignore existing target for symlink if it is already a symlink
Also check that existing symlink has desired target before allowing it.
If symlink already exists, clobber it with current one
Add 'unsymlink command to remove the symlink
Merge pull request #4692 from takluyver/setup-symlink-improvements

setup.py symlink improvements