MFC r228822:

MFC r228822:

  When building the kernel with clang, it produces several warnings which
  might be useful in some cases, but which are not severe enough to error
  out the whole kernel build.  Display them anyway, so there is at least
  some incentive to fix them eventually.

  Start with -Wtautological-compare warnings.  These usually occur when
  people check if unsigned quantities are negative, or similar cases.  To
  clean these up would be painful, and might give problems if the base
  type which is compared against changes to signed later on.

MFC r228841:

  Amend r228822 by not directly adding to CWARNFLAGS, but to an optional
  CWARNEXTRA variable, which gets included into the initial CWARNFLAGS
  setting.  This makes it easier to override CWARNFLAGS with completely
  custom settings (including enabling any disabled warnings).

  Reminded by:	arundel

MFC r228860:

  Make another clang warning, -Wempty-body, non-fatal during kernel
  builds.  All the instances of this warning in our tree are completely
  harmless.  (Most of the empty bodies look to be used simply as reminder
  for the developer to add something later.)

  While here, assign to CWARNEXTRA with ?=, so it can be overridden
  easily, if needed.

MFC r228867:

  Make another clang warning, -Wparentheses-equality, non-fatal during
  kernel builds.  All the instances of this warning in our tree are
  completely harmless, and many people seem to like adding extra
  parentheses to make precedence clearer.

git-svn-id: svn://svn.freebsd.org/base/stable/9@229278 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f