Revert "darwin: assume pthread_setname_np() is available"
Revert "darwin: assume pthread_setname_np() is available"

This reverts commit bee1bf5dd7de8da316821c32411425f7cf7ab49c.

This is necessary in order to be able to revert commit 038eacfbf4
("darwin: speed up uv_set_process_title()") from October.

PR-URL: https://github.com/libuv/libuv/pull/2568
Reviewed-By: Michael Dawson <241bf52ebd97cc4ec83d57b04d1f9e9053df032e@ca.ibm.com>
Reviewed-By: Richard Lau <8193d74c06330af7778ef55510160e4e561ed6a1@uk.ibm.com>
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

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