* 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.hh

@@ -84,6 +84,8 @@ bool pathExists(const Path & path);
    in any way canonicalised. */
 Path readLink(const Path & path);
 
+bool isLink(const Path & path);
+
 /* Read the contents of a directory.  The entries `.' and `..' are
    removed. */
 Strings readDirectory(const Path & path);