Make internal $V flag to store @ when pretty printing

Make internal $V flag to store @ when pretty printing

Verbosity flag ($V variable) let the user decide if he want to see nice
small messages ($V empty) or the standard make output displaying the raw
commands as they are executed ($V non-empty).

Now the meaning of this flag is reversed (and stores "@" when non-empty)
to let the Makefile writer to easily silence commands dependending on the
verbosity flag. When writing a rule, just prepend $V to each action you
want to conditionally silence depending on the verbosity flag.