Rails - Apr 2011 - issue installing gem Capybara on Debian5 w ruby 1.9.2 ( Rails 3.0.5) => installing native extension

On this server I already installed it in the gemsets for Ruby 1.8.7
and Rails 2.3.11 without any problem ...
but trying (many times .. ) to install in Ruby 1.9.2 (Rails 3.0.5)
raises an error ... It''s not related to the gem itself as the gem
doesn(t build any native extension .. but to one dependency...
I don''t know where to start looking for ...

here is the log message :
rubyrails:~ (ruby-1.9.2@rails3)$ gem install capybara
Fetching: nokogiri-1.4.4.gem (100%)
Building native extensions.  This could take a while...
Fetching: culerity-0.2.15.gem (100%)
Fetching: celerity-0.8.9.gem (100%)
Fetching: json_pure-1.5.1.gem (100%)
Fetching: rubyzip-0.9.4.gem (100%)
Fetching: ffi-1.0.7.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing capybara:
	ERROR: Failed to build gem native extension.

        rake RUBYARCHDIR=/home/kadoudal/.rvm/gems/ruby-1.9.2-
p180@rails3/gems/ffi-1.0.7/lib RUBYLIBDIR=/home/kadoudal/.rvm/gems/
ruby-1.9.2-p180@rails3/gems/ffi-1.0.7/lib
/home/kadoudal/.rvm/rubies/ruby-1.9.2-p180/lib/ruby/site_ruby/1.9.1/
rubygems.rb:388:in `bin_path'': can''t find gem rake
([">= 0"]) with
executable rake (Gem::GemNotFoundException)
	from /home/kadoudal/.rvm/gems/ruby-1.9.2-p180/bin/rake:19:in
`<main>''

need some clue to try solving this issue ...

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