title: Attempting to build a cython extension to a python package, not creating shared object (.so) file
body: <p>I have attempted to use the answer <a href="https://stackoverflow.com/questions/32528560/using-setuptools-to-create-a-cython-package-calling-an-external-c-library">here</a> to add the building of a cython extension into my package. It currently cythonizes the code to produce a .c file from the .pyx file but doesn't create a shared object .so file, as such when I try to import the package, and one of the modules attempts to import the shared object file it cannot find it.</p>
  
  <p>My setup.py file (this is slightly cut-down) is like so:</p>
  
  <pre><code>from setuptools import setup
  from setuptools.extension import Extension
  import os
  import numpy
  from Cython.Build import cythonize
  
  mypackage_root_dir = os.path.dirname(__file__)
  with open(os.path.join(mypackage_root_dir, 'requirements.txt')) as requirements_file:
      requirements = requirements_file.read().splitlines()
  
  extensions = [Extension(
      name="package.submodule.foo",
      sources=["package/submodule/foo.pyx"],
      include_dirs=[numpy.get_include()],
      )
  ]
  
  setup(name='package',
        version=0.1,
        description='...',
        author='my name',
        author_email='my email',
        url="...",
  
        include_package_data=True,
        packages=['package',
                  'package.submodule1',
                  'package.submodule2',
                  'package.submodule', # the one that uses the pyx file
        ],
        ext_modules = cythonize(extensions),
        install_requires=requirements,
  )
  </code></pre>
  
  <p>How can I fix this such that I can get the setup.py file to build the shared object file when <code>python setup.py install</code> is run?</p>