I have the same issue with sup-0.21:

[...]
Building native extensions.  This could take a while...
/usr/bin/ruby mkrf_conf_xapian.rb
xapian: platform specific dependencies..
xapian: installing xapian-ruby..
ERROR:  Error installing sup:
ERROR: Failed to build gem native extension.

    Building has failed. See above output for more information on the 
failure.
rake failed, exit code 1

The patch posted has been merged before 0.21 so I don't think it helped. 
I'm on Arch Linux using Ruby 2.2.0.

Did anyone get it to work?

On Tuesday, January 6, 2015 at 11:13:26 AM UTC+1, gui...@netcourrier.com 
wrote:
>
> Hello all,
>
> I updated my system to ruby 2.2 and I can't get sup to work.
> At first it didn't work so I figured I needed to recompile it, so I 
> removed and reinstalled it. However, simply typing 'gem install sup' fails 
> with this message:
>
> Fetching: sup-0.20.0.gem (100%)
> Building native extensions.  This could take a while...
> ERROR:  Error installing sup:
>     ERROR: Failed to build gem native extension.
>
>     /usr/bin/ruby mkrf_conf_xapian.rb
> xapian: installing xapian-ruby..
> xapian: platform specific dependencies..
>
> rake failed, exit code 1
>
> Gem files will remain installed in 
> /home/guiniol/.gem/ruby/2.2.0/gems/sup-0.20.0 for inspection.
> Results logged to 
> /home/guiniol/.gem/ruby/2.2.0/extensions/x86_64-linux/2.2.0/sup-0.20.0/gem_make.out
>
>
> I figured the problem was with xapian, so I tried to first install 
> xapian-ruby and then sup, which works. However, sup doesn't launch. I get 
> this error instead:
>
> /usr/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require': 
> cannot load such file -- dl/import (LoadError)
>     from /usr/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in 
> `require'
>     from /home/guiniol/.gem/ruby/2.2.0/gems/sup-0.20.0/bin/sup:111:in 
> `<module:Redwood>'
>     from /home/guiniol/.gem/ruby/2.2.0/gems/sup-0.20.0/bin/sup:74:in `<top 
> (required)>'
>     from /home/guiniol/.gem/ruby/2.2.0/bin/sup:23:in `load'
>     from /home/guiniol/.gem/ruby/2.2.0/bin/sup:23:in `<main>'
>
>
> Has anyone else tried sup with ruby 2.2? Is there anything I can try to 
> get it to work?
>
> Cheers,
>
> Guillaume
>