This cleans up the 'cmake --build' command-line interface:
- Rename --clean to --clean-first to better describe it.
- Replace --extra-options with a -- separator to simplify passing of
multiple native build tool options.
- Document the options in the main CMake man page description of the
--build option, and shares this with the usage message.
- Require --build to be the first argument when present.
- Move implementation into cmakemain where it belongs.
Brad King
Bill Hoffman