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/ruby21 ${PREFIX}/bin/ruby
 ln -sf ${PREFIX}/bin/erb21 ${PREFIX}/bin/erb
 ln -sf ${PREFIX}/bin/irb21 ${PREFIX}/bin/irb
 ln -sf ${PREFIX}/bin/rdoc21 ${PREFIX}/bin/rdoc
 ln -sf ${PREFIX}/bin/ri21 ${PREFIX}/bin/ri
 ln -sf ${PREFIX}/bin/rake21 ${PREFIX}/bin/rake
 ln -sf ${PREFIX}/bin/gem21 ${PREFIX}/bin/gem
 ln -sf ${PREFIX}/bin/testrb21 ${PREFIX}/bin/testrb
