Update Python packaging for packages containing C libs

[veit]

• answer.c

  int answer(void) { return 42; }

• answer.h

  int answer(void);

• answer.pxd

  cdef extern from "answer.h":
      int answer()

• answer.pyx

  cimport answer
  cpdef test():
      return answer()

• setup.py

  import sys
  from distutils.core import setup
  from distutils.command.build_clib import build_clib
  from distutils.extension import Extension
  from Cython.Distutils import build_ext
  
  libanswer = ('answer', {'sources': ['answer.c']})
  
  ext_modules=[
      Extension("demo", ["answer.pyx"])
  ]
  
  def main():
      setup(
          name = 'answer',
          libraries = [libanswer],
          cmdclass = {'build_clib': build_clib, 'build_ext': build_ext},
          ext_modules = ext_modules
      )
  
  if __name__ == '__main__':
      main()

• test.py

  import answer
  
  def test_answer():
      ret = demo.test()
      assert(ret == 42)
  
  if __name__ == '__main__':
      test_answer()

• Build the C library:

  $ python setup.py build_clib
  

• Build the extension module in place

  $ python setup.py build_ext --inplace