Fix installation from git

If pystencils is installed from git and cython is not installed you get the error: gcc: error: pystencils/boundaries/createindexlistcython.c: No such file or directory. This is because the generated C file is not shipped and thus it is not available.

On pypi the C-file is shipped and thus it works.

Fixes #14 (closed)

setup.py

@@ -59,12 +59,14 @@ def readme():
 
 def cython_extensions(*extensions):
     from distutils.extension import Extension
-    ext = '.pyx' if USE_CYTHON else '.c'
-    result = [Extension(e, [e.replace('.', '/') + ext]) for e in extensions]
     if USE_CYTHON:
+        ext = '.pyx'
+        result = [Extension(e, [e.replace('.', '/') + ext]) for e in extensions]
         from Cython.Build import cythonize
         result = cythonize(result, language_level=3)
-    return result
+        return result
+    else:
+        return None
 
 
 try: