* nix-build: use an indirection scheme to make it easier for users to

get rid of GC roots.  Nix-build places a symlink `result' in the
  current directory.  Previously, removing that symlink would not
  remove the store path being linked to as a GC root.  Now, the GC
  root created by nix-build is actually a symlink in
  `/nix/var/nix/gcroots/auto' to `result'.  So if that symlink is
  removed the GC root automatically becomes invalid (since it can no
  longer be resolved).  The root itself is not automatically removed -
  the garbage collector should delete dangling roots.

src/libutil/util.cc

@@ -135,6 +135,15 @@ Path readLink(const Path & path)
 }
 
 
+bool isLink(const Path & path)
+{
+    struct stat st;
+    if (lstat(path.c_str(), &st))
+        throw SysError(format("getting status of `%1%'") % path);
+    return S_ISLNK(st.st_mode);
+}
+
+
 Strings readDirectory(const Path & path)
 {
     Strings names;