Ensure that Perl processes delete their entry in the temproots directory

By moving the destructor object to libstore.so, it's also run when
download-using-manifests and nix-prefetch-url exit.  This prevents
them from cluttering /nix/var/nix/temproots with stale files.

src/libstore/gc.cc

@@ -214,6 +214,18 @@ void removeTempRoots()
 }
 
 
+/* Automatically clean up the temporary roots file when we exit. */
+struct RemoveTempRoots 
+{
+    ~RemoveTempRoots()
+    {
+        removeTempRoots();
+    }
+};
+
+static RemoveTempRoots autoRemoveTempRoots __attribute__((unused));
+
+
 typedef boost::shared_ptr<AutoCloseFD> FDPtr;
 typedef list<FDPtr> FDs;