No warning when strace is not present for parallel builds.

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I spent a while trying to figure out why my parallel build was not working 
inside a docker container when it worked fine outside the container. The 
ultimate reason was that strace was not present. I think it would make sense to 
provide a warning when strace is not found but "parallel_ok=True". I can submit 
a patch for this, but it seems like that'd be more effort than this is worth.

Original issue reported on code.google.com by [e-mail] on 15 Nov 2014 at 10:00