warning for -Wformat and non-literal-string format

[Gavin Romig-Koch]

Hey Ken, long time...

At minimum I think you need to change the doc to say that -Wformat
will warn if the format string isn't a string literal.

My preference would be to have your new functionality controled
by an option; perhaps -Wformat continues to do what it always did,
and -Wformat-all would also print your new warning.  But I'm open
to other opinions.

(Oh, and does the FSF have an assignment from you?)

                                       -gavin...