FIX: flycheck-clang-language-standard

As stated in https://github.com/flycheck/flycheck/issues/333 by
@lunaryorn

Do not change flycheck-clang-language-standard globally, but change it
via file or directory variables, or set it in a mode-hook, e.g.:

(add-hook 'c++-mode-hook
  (lambda () (
     setq flycheck-clang-language-standard "c++11")))