mkmf LoadError on Solaris 11

Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

        /opt/csw/bin/ruby18 extconf.rb 
extconf.rb:1:in `require': no such file to load -- mkmf (LoadError)
        from extconf.rb:1

If you’re using OpenCSW, make sure you’ve got the ruby18_dev package installed.