configure: Fix call without options

Previously this would result in config.opt file not being written.

Change-Id: I9ce349a7cda9f399fa789a569e46138ea90769f5
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

configure

@@ -928,7 +928,7 @@ if [ "$BUILD_WITH_CMAKE" = "yes" ]; then
     optfilename=config.opt
     if [ -z "$optfile" ]; then   # only write optfile if not currently redoing
         optfilepath=${outpathPrefix}${optfilename}
-        if [ -f "$optfilepath" ]; then rm "$optfilepath"; fi
+        > "$optfilepath"
         for arg in "$@"; do
             if [ "$arg" = "-top-level" ]; then
                continue