Build error using CMake at macOS Sierra

Here's the build log:

```
[ 35%] Building CXX object absl/time/CMakeFiles/time_zone.dir/internal/cctz/src/time_zone_format.cc.o
In file included from /Users/lyy/workspace/C++/abseil-cpp/absl/time/internal/cctz/src/time_zone_format.cc:27:
In file included from /Users/lyy/workspace/C++/abseil-cpp/absl/time/internal/cctz/include/cctz/time_zone.h:28:
In file included from /Users/lyy/workspace/C++/abseil-cpp/absl/time/internal/cctz/include/cctz/civil_time.h:18:
In file included from /Users/lyy/workspace/C++/abseil-cpp/absl/time/internal/cctz/include/cctz/civil_time_detail.h:20:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/ostream:138:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/ios:216:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/__locale:18:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/mutex:189:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/__mutex_base:17:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/__threading_support:156:1: error: unknown
type name 'mach_port_t'
mach_port_t __libcpp_thread_get_port();
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/__threading_support:300:1: error: unknown
type name 'mach_port_t'
mach_port_t __libcpp_thread_get_port() {
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/__threading_support:301:12: error: use of
undeclared identifier 'pthread_mach_thread_np'
return pthread_mach_thread_np(pthread_self());
^
3 errors generated.
make[2]: *** [absl/time/CMakeFiles/time_zone.dir/internal/cctz/src/time_zone_format.cc.o] Error 1
make[1]: *** [absl/time/CMakeFiles/time_zone.dir/all] Error 2
make: *** [all] Error 2
```

Saw similar issue elsewhere: https://github.com/tensorflow/tensorflow/issues/13220