[Technology] emacs and vala-mode: set-difference

When using emacs 24.5 and vala-mode, I started getting this error:
"Symbol's function definition is void: set-difference"

After some googling, it turns out to be related to cc-mode using cl (CommonLisp).  The not-ideal workaround for now is to apparently include

(require 'cl)

In your ~/.emacs file.