gtlsfiledatabase-gnutls: Succeed even without an anchor file

This way, it will produce an empty default database. Otherwise, there
will be no default database and that shoud always succeed.

https://bugzilla.gnome.org/show_bug.cgi?id=727282
configure: Don't print a warning if the global CA anchor file is disabled

https://bugzilla.gnome.org/show_bug.cgi?id=727282