Eric,

Thanks. 

See the pull request for why this might qualify for a back port: other parts of distutils recognise '.mm' as a valid extension, but '.mm' doesn't work yet because UnixCCompiler doesn't support this yet. 

On the other hand this is effectively a new feature because '.mm' has never worked, and there is an easy workaround (monkey patching the compiler class).

And writing this I agree that this patch should not be back ported.