In commit v3.3.0-rc1~49^2~2 (cmake-gui: Add --install option to add
command-line tools on OS X, 2015-05-19) the option default was set to
/usr/bin because that is where the old command line install dialog
placed the symlinks.  A better default is /usr/local/bin because it is
meant for locally installed software rather than Apple-installed tools.
Also, as of OS X El Capitan, special privileges are required even for
root to modify /usr/bin but not /usr/local/bin.