MFC r256915
Stop conflating WITHOUT_CLANG with WITHOUT_CLANG_IS_CC. This allows
bootstrapping a copy of clang without building clang for the base system
which is useful for nanobsd and similar setups. It's still probably
wrong to conflate what is installed as /usr/bin/cc with the selection
of a bootstrap compiler under WITH*_CLANG_IS_CC, but that's for another
day.
Sponsored by: DARPA/AFRL
Approved by: re (gjb)
git-svn-id: svn://svn.freebsd.org/base/stable/10@257449
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