Build: configure: surpriseless --with{,out}-gnutls-priorities handling

Avoiding evaluating underlying definition as "no" (will just fall
back to the designated default, instead), sticking with --without-brand
as a precedence.

Also an empty string propagated from there (--with-gnutls-priorities="")
would result in a run-time error:

$ gnutls-cli -l --priority=":+DHE-PSK:+PSK"
> Cipher suites for :+DHE-PSK:+PSK
> Syntax error at: :+DHE-PSK:+PSK