More header fixes

[nalimilan]

The first commit fixes a warning on Fedora rawhide with GCC6. But we should check that this works on Windows.

The second commit is just a cleanup.

[nalimilan]

I've enabled OS X on Travis CI in a branch, and the build works (though tests fail): https://travis-ci.org/JuliaLang/openlibm/jobs/113449199

But there's still another warning about redefining __pure2, which is already defined by sys/cdefs.h. I'm not sure what we should do here: define it only if not yet defined, so that we trust the system to do what's right? Or call it __openlibm_pure?

[ViralBShah]

I would prefer to do the conditional define. Changing these things more means more difficulty in importing upstream changes.

[ViralBShah]

The failing cases are all 1 ulp and some FP exception stuff. I would say that if no new failures arise, we should be ok.

[ViralBShah]

If we can address the __pure2 stuff, let's get this in for the 0.5 release. Nice to have but not essential.

[nalimilan]

I'm preparing a fix.

[nalimilan]

With this small patch it should be good to go. I agree the errors can wait for another release since they aren't regressions.