Forcibly cast pthread_t to tid (long)
Forcibly cast pthread_t to tid (long)

Summary:
This was actually an unsafe use of static_cast<>
and MacOS's compiler (rightly) complains about it.

Fallback on a forceful cast for now, but someone should
possibly rethink this design.

Test Plan: Helps Mac build, doens't hurt linux build

Reviewed By: joelm@fb.com

Subscribers: alandau, bmatheny, mshneer, yfeldblum

FB internal diff: D1875373

Signature: t1:1875373:1425061392:0917da362262c8eec77524cd18d4cf254a19be37

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