Don't copy files for clean target

[sgillies]

Debian builds were broken by this regression.

[sebastic]

Because ext_modules is defined in the the if/elif blocks, if none of the conditions match the variable remains undefined causing an error:

$ python2.7 setup.py clean 
Traceback (most recent call last):
  File "setup.py", line 248, in <module>
    ext_modules=ext_modules,
NameError: name 'ext_modules' is not defined

This was explicitly handled by my PR, see: setup.py line 180