title: build scipy error cythonize failed
body: <p>I'm trying to build scipy and I get a RuntimeError:</p>
  
  <pre><code>$sudo python setup.py build
  Processing scipy/cluster/_vq_rewrite.pyx
  Traceback (most recent call last):
    File "tools/cythonize.py", line 172, in &lt;module&gt;
      main()
    File "tools/cythonize.py", line 168, in main
      find_process_files(root_dir)
    File "tools/cythonize.py", line 160, in find_process_files
      process(cur_dir, fromfile, tofile, function, hash_db)
    File "tools/cythonize.py", line 137, in process
      processor_function(fromfile, tofile)
    File "tools/cythonize.py", line 66, in process_pyx
      raise OSError('Cython needs to be installed')
  OSError: Cython needs to be installed
  
  Traceback (most recent call last):
    File "setup.py", line 209, in &lt;module&gt;
      setup_package()
    File "setup.py", line 202, in setup_package
      generate_cython()
    File "setup.py", line 147, in generate_cython
      raise RuntimeError("Running cythonize failed!")
  RuntimeError: Running cythonize failed!
  </code></pre>
  
  <p>What are the ways to provide cython to remedy this error?</p>