[news.eclipse.tools] How do I specify an external directory for classes?

[x@xxxxxxx (Cedric Beust)]

I need to specify that my project should be built using a certain 
"external" (not under my project) directory in the classpath, but it looks 
like Java Build Path/Libraries only lets me specify

- External jar files
- Internal jar files
- Internal directories (below my project)
- Variables (I tried to define one to point to this external directory but 
Eclipse doesn't seem to take it into account)

How do I add e:\foo\classes in the classpath of my project?

-- 
Cedric