Fix a problem running build_ext -i w/no extensions.

--HG--
branch : setuptools
extra : convert_revision : svn%3A6015fed2-1504-0410-9fe1-9d1591cc4771/sandbox/trunk/setuptools%4041217

setuptools/command/build_ext.py

@@ -20,7 +20,7 @@ class build_ext(_build_ext):
 
     def copy_extensions_to_source(self):
         build_py = self.get_finalized_command('build_py')
-        for ext in self.extensions:
+        for ext in self.extensions or ():
             fullname = ext.name
             modpath = fullname.split('.')
             package = '.'.join(modpath[:-1])