Use qtbase optimize_size configure option

The optimize-size configure option got landed generally in qtbase, so
respond to that instead of doing our own.

Change-Id: I8bfcf494a6f0d599bf375ae4ea73ce86ee02216b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>