Remove superfluous pthread flag on OS X.

[david-german-tri]

Contributes to #5183.

@liangfok for feature review

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This change is 

[liangfok]

When I first tested this using a dirty workspace, the pthread warnings persisted. However, after cleaning the workspace using bazel clean --expunge, the warnings continued to show up. Did I do something wrong? I am using 559ee99.

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Comments from Reviewable

[liangfok]

Here is what I see: https://gist.github.com/liangfok/c194d35efff74d8249ff2c5ad6a9d5c3

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Review status: 0 of 1 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[liangfok]

Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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[david-german-tri]

Yep, I missed some - thanks! done.

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Review status: 1 of 3 files reviewed at latest revision, all discussions resolved.

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[liangfok]

I tested this on my OSX machine and it works perfectly. Thanks!

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Reviewed 2 of 2 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[david-german-tri]

+@jwnimmer-tri platform

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Review status: all files reviewed at latest revision, all discussions resolved.

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[jwnimmer-tri]

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Review status: all files reviewed at latest revision, 1 unresolved discussion.

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a discussion (no related file):
BTW So this is fine, but now that tools/otherlib.BUILD files are using @//tools:linux conditions, they are not really separable from Drake anyway, so I wonder if a deps = ["@//tools:pthread"] (and a cc_library rule with the flag intools/BUILD) in lieu of linkopts=... would be worth it to de-dup the select logic?

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Comments from Reviewable