fprintftime: wrap macro code argument in "do {...} while(0)"

fprintftime: wrap macro code argument in "do {...} while(0)"

* lib/strftime.c (cpy) [FPRINTFTIME]: The second argument to the
cpy macro must be a statement that can be followed by a semicolon.
Now that the else clause contains a comment and is hence longer
than one line, I require curly braces.  That in turn requires
that we wrap this code block in the standard do...while(0).