ARROW-1139: Silence dlmalloc warning on clang-4.0

[pcmoritz]

An alternative would be to patch dlmalloc similarly to https://github.com/greg7mdp/dlmalloc

[cpcloud]

@pcmoritz @wesm

I just pulled this down and built plasma and successfully ran all the tests.

+1 here, thanks for doing this!

couple of things:

• It might be nice to eventually get rid of the warnings (though it looks like this source might be out of our control).
• There are some *_generated.h files laying around after the build, @pcmoritz can you add a .gitignore file to the src/plasma/format directory?
• cmake throws a warning about flatbuffers_ep. we should allow users to build plasma with flatbuffers code in $FLATBUFFERS_HOME. I can follow up with a patch to do that.

[wesm]

I'm going to merge and leave these items to follow up patches (I also noticed the _generated.h files)

[pcmoritz]

Thanks for testing this! I created a JIRA for .gitignore here and will make a patch: https://issues.apache.org/jira/browse/ARROW-1146

I'm not as sure what is involved with flatbuffers_ep, @cpcloud if you know what is the right way to do this it might be faster if you submit a patch.