Improve fallback upon lack of C++11 thread_local specifier
- removed configure-time check that defines VTK_HAS_THREADLOCAL
- introduced new VTK_THREAD_LOCAL macro in vtkSetGet.h that wraps raw thread_local keyword
- replaced all uses of thread_local with VTK_THREAD_LOCAL
- fallback to _Thread_local keyword with Xcode < 8