nullptr issue in Mac Apple Clang

[AbdealiLoKo]

When compiling rabit in Apple's Clang compiler I seem to be getting this error:

In file included from rabit/src/engine.cc:17:
rabit/src/./thread_local.h:43:36: error: initializer for thread-local variable must be a constant
      expression
    static MX_TREAD_LOCAL T* ptr = nullptr;
                                   ^~~~~~~
/Library/Developer/CommandLineTools/usr/include/c++/v1/__nullptr:49:17: note: expanded from macro 'nullptr'
#define nullptr _VSTD::__get_nullptr_t()
                ^~~~~~~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/usr/include/c++/v1/__config:392:15: note: expanded from macro '_VSTD'
#define _VSTD std::_LIBCPP_NAMESPACE
              ^

This is occurring with latest master branch

[hcho3]

Closing, as Rabit have been moved into dmlc/xgboost. See discussion in dmlc/xgboost#5995.