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 after: 2 weeks