* Applied rbroek's patch from the branch at

https://svn.cs.uu.nl:12443/repos/trace/buildfarm-control/trunk/ext/nix/,
  with some modifications.  This allows `nix-env -qa' to show the
  attribute path that can be used to unambiguously install a package
  using `nix-env -i -A'.  Example:

    $ nix-env -f top-level/all-packages.nix -qaA subversion xorg-server
    subversionWithJava  subversion-1.2.3
    subversion          subversion-1.3.2
    subversion14        subversion-1.4.0pre-rc1
    xorg.xorgserver     xorg-server-1.1.0

src/libexpr/get-drvs.hh

@@ -20,6 +20,7 @@ private:
     
 public:
     string name;
+    string attrPath; /* path towards the derivation */
     string system;
 
     shared_ptr<ATermMap> attrs;