* 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/libstore/gc.hh

@@ -27,7 +27,8 @@ void addTempRoot(const Path & path);
 void removeTempRoots();
 
 /* Register a permanent GC root. */
-Path addPermRoot(const Path & storePath, const Path & gcRoot);
+Path addPermRoot(const Path & storePath, const Path & gcRoot,
+    bool indirect);
 
 
 #endif /* !__GC_H */