It is now only possible to prescribe options for
subprocesses (e.g. build_ext) in the file setup.cfg
But some of these options are obligatory to get the correct 
result for the action.
These options can be set in the file setup.cfg, but it is not
clear that these options should not be changed, nor is it
clear in the setup.py file that the setup.cfg is relevant.
I tend now to not to use the setup.cfg and
call 'python setup.py' (without arguments).
The setup.py will then fill in the necessary options in 
sys.argv .
It would be much nicer if these options could be explicitly 
given as defaults for the subprocess.