> Failure to build _multiprocessing will mean that multiprocessing cannot 
> be imported.  So if the function goes somewhere in multiprocessing then 
> it makes running the test suite with multiple processes dependent on the 
> building of _multiprocessing.  Not sure if that is much of a problem
> since one can just run the test suite normally.

I don't think that's a problem indeed.

> Also I don't think "-i" (inspect/interactive) should be passed on: a 
> child process's stdin is os.devnull.

Ah, you're right.

> BTW, why isn't the use of "-u" (unbuffered stdout, stderr) reflected in 
> sys.flags?

Don't know. sys.flags was introduced in issue1816, long after the -u flag itself. The code to reflect "-u" is commented out, perhaps Christian Heimes can shed some light on this.