Updated npth to 1.3.

Note: this now needs "-pthread" on NetBSD; please check if it's needed
on your OPSYS too.

Noteworthy changes in version 1.3 (2016-11-22)
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* Bypass npth_protect/npth_unprotect iff the library has not yet been
  initialized.

* Fix problems on macOS and AIX

* Improve detection of clock_gettime

(wiz)