hi all:
when i install mongrel

Building native extensions.  This could take a while...
extconf.rb:1:in `require': no such file to load -- mkmf (LoadError)
        from extconf.rb:1

ERROR:  Error installing gem mongrel-0.3.13.3.gem[.gem]: ERROR: Failed
to build gem native extension.
Gem files will remain installed in
/usr/lib/ruby/gems/1.8/gems/mongrel-0.3.13.3 for inspection.