configurator: Fix warning in HAVE_FOR_LOOP_DECLARATION

Visual C++ prints "warning C4702: unreachable code" due to the return
statement after the for loop which is never reached.  Fix this by
setting a variable returned by a single return statement at the end.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>