configure: touch scanner.rl if --enable-fastparser

It is not required to delete original scanner.c now, if you want to generate fast parser.

Change-Id: Ifbbc5a6a596b2d9f2238b53d117c86535bb4d111
rrset-test: fix compile warning

Change-Id: Ie3849686302de8ba68e1c72aeb6558ee36b49176