Ensure sandbox link to local .emacs.d directory is created properly

If `sandbox/.emacs.d` already exists, calling `ln` again seems to create links
within this directory.  Ensure that this does not happen anymore by deleting the
link first if it already exists.  Also add `-T` to `ln` to ensure its arguments
are interpreted properly.

They day I understand argument interpretation in `ln` will be a good day …

Makefile

@@ -26,7 +26,8 @@ distclean: clean
 
 sandbox-start:
 	mkdir -p sandbox
-	ln -s $(PWD) sandbox/.emacs.d
+	rm -f sandbox/.emacs.d
+	ln -sT $(PWD) sandbox/.emacs.d
 	HOME=$(PWD)/sandbox emacs
 
 timelinetools-test.elc: timeline-tools.elc