Fix undefined ext_modules introduced via Toblerity#362

See: Toblerity#362 (comment)

setup.py

@@ -155,6 +155,7 @@ def run(self):
     extra_link_args=extra_link_args)
 
 # When building from a repo, Cython is required.
+ext_modules = []
 if source_is_repo and "clean" not in sys.argv:
     log.info("MANIFEST.in found, presume a repo, cythonizing...")
     if not cythonize: