vars.am: Tweak `silent-rules' machinery.

vars.am: Tweak `silent-rules' machinery.

Since Automake 1.11, the advice for setting up custom silent-rules
recipes has changed, so use the new machinery.

Also, I'm no longer mainly working on wheezy, and Automake has made the
operation field two spaces wider while I wasn't looking, so make the
output line up properly.

This means that tripe now requires Automake 1.11.2 or later to build from
the Git tree.