C:\Windows\system32>igem install rails
Building native extensions. This could take a while...
ERROR: Error installing rails:

    ERROR: Failed to build gem native extension.

 
C:/IronRuby/bin/ir.exe extconf.rb
C:/IronRuby/Lib/ruby/1.9.1/mkmf.rb:9:in `const_missing': uninitialized constant
RbConfig::MAKEFILE_CONFIG (NameError)

    from C:/IronRuby/Lib/ruby/1.9.1/mkmf.rb:9
    from C:/IronRuby/Lib/ruby/1.9.1/rubygems/custom_require.rb:29:in `requir

e'

    from C:/IronRuby/Lib/ruby/1.9.1/rubygems/custom_require.rb:29:in `requir

e'

    from extconf.rb:1

 
 
 
Gem files will remain installed in C:/IronRuby/Lib/ruby/gems/1.9.1/gems/json-1.6
.5 for inspection.
Results logged to C:/IronRuby/Lib/ruby/gems/1.9.1/gems/json-1.6.5/ext/json/ext/p
arser/gem_make.out