Makefile: m4_changequote([[[, ]]]) to avoid accidental quoting.

The default m4 quote characters: `QUOTE' are troublesome, because ` is
fairly likely to pop up in a shell script (if not for a subshell, because
it is a useful character in comments and user-facing messages).

So, this changes it to [[[QUOTE]]], as it is unlikely to see three braces
together like that, let alone in unbalanced sets.