Redmine

Hello,

I am trying to install Redmine on my RaspberryPi by using this HowTo:
http://www.redmine.org/projects/redmine/wiki/HowTo_Install_Redmine_210_on_Debian_Squeeze_with_Apache_Passenger

But when I try to install the gem bundle I get an error...

pi@raspberrypi /usr/local/share/redmine $ sudo bundle install --without development test postgresql sqlite
Please configure your config/database.yml first
Fetching gem metadata from https://rubygems.org/.........
Fetching additional metadata from https://rubygems.org/..
Resolving dependencies........................................................................................
Installing rake (10.1.1)
Installing i18n (0.6.9)
Installing multi_json (1.8.4)
Installing activesupport (3.2.16)
Installing builder (3.0.0)
Installing activemodel (3.2.16)
Installing erubis (2.7.0)
Installing journey (1.0.4)
Installing rack (1.4.5)
Installing rack-cache (1.2)
Installing rack-test (0.6.2)
Installing hike (1.2.3)
Installing tilt (1.4.1)
Installing sprockets (2.2.2)
Installing actionpack (3.2.16)
Installing mime-types (1.25.1)
Installing polyglot (0.3.3)
Installing treetop (1.4.15)
Installing mail (2.5.4)
Installing actionmailer (3.2.16)
Installing arel (3.0.3)
Installing tzinfo (0.3.38)
Installing activerecord (3.2.16)
Installing activeresource (3.2.16)
Using bundler (1.5.3)
Installing coderay (1.1.0)
Installing rack-ssl (1.3.3)

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

        /usr/bin/ruby1.9.1 extconf.rb 
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
    from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
    from extconf.rb:1:in `<main>'

Gem files will remain installed in /var/lib/gems/1.9.1/gems/json-1.8.1 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/json-1.8.1/ext/json/ext/generator/gem_make.out
An error occurred while installing json (1.8.1), and Bundler cannot continue.
Make sure that `gem install json -v '1.8.1'` succeeds before bundling.

I am not sure what I did wrong or what I am still doing wrong...
I tried to install the json gem manually, but without any success.

Does someone know the solution for this problem? Or maybe there is a good HowTo for Redmine on RaspberryPi?

Thank you!