Makefile: Magic silent rules.

Use the facilities of GNU make to create a magic function that will
on the first invocation print a description of how to enable verbose
compile lines and then print the quiet rule.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Carl Worth <cworth@cworth.org>
Cc: Mikhail Gusarov <dottedmag@dottedmag.net>
[ickle: Rebased, and duplicate command string eliminated.]
[ickle: Fixed verbose bug pointed out by Mikhail]