mkmf LoadError on Solaris 11

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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.