System Info: macOS 10.11, Xcode 8 with macOS 10.12 SDK
libZMQ version: commit 2e92643

Problem:

• Prior to macOS 10.12 clock_gettime was not defined, requiring a custom implementation in libZMQ.
• When using macOS 10.12 libzmq should use the system clock_gettime, but when targeting 10.11 or earlier it should use the custom implementation.

How to Reproduce:

• Install Xcode 8 on macOS 10.11, then build and run tests
• Target 10.11 when building on macOS 10.12

Tests will fail with dyld: lazy symbol binding failed: Symbol not found: _clock_gettime

Solution:
Rename the custom implementation of clock_gettime for macOS and wrap all usage in preprocessor macros to only enable the alternative version when using macOS 10.11 or earlier.