MFC 232322:

MFC 232322:
  Add a WITH_CLANG_IS_CC option for src.conf(5), disabled by default, that
  installs clang as /usr/bin/cc, /usr/bin/c++ and /usr/bin/cpp.

  Note this does *not* disable building and installing gcc, which will
  still be available as /usr/bin/gcc, /usr/bin/g++ and /usr/bin/gcpp.  If
  you want to disable gcc completely, you must use WITHOUT_GCC.

MFC 232323:
  Regenerate src.conf(5) after r232322.

MFC 232477:
  In r232322, I forgot one case where a check for MK_CLANG_IS_CC was
  needed, in sys/conf/kern.pre.mk.  Add it now.

MFC 232522:
  Fix a thinko in r232322, where gcc (and its tools) are not built during
  the cross-tools stage, if CC=clang and WITH_CLANG_IS_CC is not set.

  This causes no 'cc' to be installed in the temporary cross-tools tree,
  making lint fall over later in the build, because it ignores ${CC} and
  attempts to run 'cc' anyway.

  To fix this, only skip building gcc during cross-tools, if WITHOUT_GCC
  is set, or if WITH_CLANG_IS_CC is set.

  Pointy hat to:	dim

git-svn-id: svn://svn.freebsd.org/base/stable/9@233099 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f