If we consider that Distutils didn't provide c++ support, and that it partially worked by accident (through gcc), I would call it a new feature. Especially since it requires a new option.

We are freezing the API so we can't add options to methods/functions. 

That's why it could be done on distutils2 instead, which will be released when 2.7 final is released (but not in the stdlib), and backward compatible with 2.4, 2.5, 2.6.

If we could do the proper job for C++ support in distutils2 that would be better. 

Notice that distutils2 is a fork of how distutils was in trunk last month, so it should be fairly simple to depend on it in your project, to benefit from new features etc. The goal is to push it back in the stdlib in 3.3. 

If you want to start a dev. branch of your project using distutils2, I am willing to help there.