BOOST_WORKAROUND(__clang_major__, ...) generates an undefined preprocessor symbol

This code in config.hpp:
```
#if !defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES) && !BOOST_WORKAROUND(BOOST_GCC, < 40900)\
      && !BOOST_WORKAROUND(BOOST_MSVC, < 1900) && !BOOST_WORKAROUND(__clang_major__, <= 4)
#  define BOOST_TT_HAS_ASCCURATE_IS_FUNCTION
#endif
```
Can generate a warning or error as __clang_major___WORKAROUND_GUARD is not defined.