[Distutils] Allow to debug extension modules

[Ivan Pozdeev]

In the 
https://mail.python.org/pipermail/python-ideas/2018-January/048558.html 
thread, it was discovered that, stumpingly enough, it seems not possible 
to debug extension modules!

* For a release Python, it's not even possible to build an extension 
without optimizations

* For a debug Python, it's possible to build but not possible to install 
it, or any other package that depends on it

Moreover, the feedback says that it makes no sense to build an extension 
with _DEBUG against a release Python and vice versa. But distutils' 
build_ext logic does (or at least trying to do since that breaks) just 
this, as it looks at the --debug option to `build' instead of checking 
the type of the running Python.

Am I missing something crucial here, or do I need to fix distutils now 
to be able to debug a problem in my extension module?

-- 
Regards,
Ivan