Update clang support.

* semantic/bovine/clang.el (doc): Mention that this file must not be
  merged to Emacs.
  (semantic-clang-show-errors): New user variable.
  (semantic-clang-system-includes): New internal variable.
  (semantic-clang-activate): Initialize it.
  (semantic-clang-possible-completions): Display errors if exit code
  is not zero.  Append `semantic-clang-system-includes' when calling
  clang.
  (semantic-clang-get-system-includes): Obtain system includes by
  compiling file with clang.
  (semantic-clang-args-from-project): Prepend project directory to
  project's includes.  Also append the project directory as an include
  directory.
(semantic-clang-version-string): Support clang version on OS X.