[tor-bugs] #20235 [Core Tor/Tor]: Configure misidentifies clock_gettime and getentropy on macOS Sierra/XCode8 when compiling for iOS 8

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#20235: Configure misidentifies clock_gettime and getentropy on macOS Sierra/XCode8
when compiling for iOS 8
-------------------------------------------------+-------------------------
 Reporter:  rainwolf                             |          Owner:
     Type:  defect                               |         Status:
                                                 |  needs_review
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.2.9.x-final
Component:  Core Tor/Tor                         |        Version:  Tor:
                                                 |  0.2.8.8
 Severity:  Normal                               |     Resolution:
 Keywords:  Sierra, XCode 8, clock_gettime,      |  Actual Points:
  getentropy, libSystem.dylib 028-backport osx   |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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Comment (by teor):

 For the record, my github branch `weak_link_getentropy` is an alternative
 solution.

 But I think it's much more complex in the clock_gettime case, and in
 general, it has more failure modes, so I would prefer nickm's solution.

 I have reviewed nickm's branch and it looks good, but I have no OS X 10.11
 or earlier to test it on.

 (To test, we need to compile on macOS 10.12 with `export
 MACOSX_DEPLOYMENT_TARGET=10.11`, then try to run the binary on an earlier
 OS X version.)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20235#comment:8>
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