Fixes for compiling without Potrace or Aspell
If potrace unavailable, hide from the context menu (backport proposed for this)
Addresses #354 (closed)
If Aspell unavailable, don't build spellcheck dialog and hide from context menu.
I've left the extra flags inside the spellcheck dialog in case someone wants to modify the dialog to be workable without Aspell.
Fixes inbox#709 (closed)
Edit: Rewritten code/description.