Doing something like:

    gen CSharp, "example.dll", :sources => sys["**/*.cs"],
                               :libs    => ["System.Web"]


will cause:

    rant: [ERROR] in file `/usr/lib/ruby/1.8/rant/rantlib.rb', line 577: 
                  in prerequisites: no such file or task: `System.Web'

I suggset to distinguish somehow, what references should be pre-depends (usually only local assemblies) and what references
shouldn't.

e.g. Diva does this in Scons by specifying assemblies as "#bin/local.dll' and 'System.Configuration' where the
'#' just indicates an assembly, that should also be a pre-dependency.