If you want to use this package as your default system ruby, as root
create symbolic links like so (overwriting any previous default):
 ln -sf ${PREFIX}/bin/ruby18 ${PREFIX}/bin/ruby
 ln -sf ${PREFIX}/bin/erb18 ${PREFIX}/bin/erb
 ln -sf ${PREFIX}/bin/irb18 ${PREFIX}/bin/irb
 ln -sf ${PREFIX}/bin/rdoc18 ${PREFIX}/bin/rdoc
 ln -sf ${PREFIX}/bin/ri18 ${PREFIX}/bin/ri
 ln -sf ${PREFIX}/bin/testrb18 ${PREFIX}/bin/testrb