Turn gnu++ off if __float128 is unsupported in clang:
Turn gnu++ off if __float128 is unsupported in clang:

      // clang currently supports native __float128 only on few targets, and
      // this target does not have it. The most visible consequence of this is a
      // specialization
      //    __is_floating_point_helper<__float128>
      // in include/c++/6.3.0/type_traits:344 that clang then rejects. The
      // specialization is protected by !if _GLIBCXX_USE_FLOAT128 (which is
      // unconditionally set in c++config.h) and #if !__STRICT_ANSI__. Tweak the
      // latter by disabling GNUMode.

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