security/gnupg: Use gpgrt-config for configure

Use gpgrt-config in the configure stage over the old individual
*-config scripts from the various gnupg-related libraries (e.g:
gpg-error-config, libassuan-config, ksba-config). Upstream has
prefered this method since version 2.3.0 released 2021-04-07 and
the old scripts are now starting to be phased-out.

This prepares for devel/npth >= 1.7 which no longer installs
npth-config by default, so gpgrt-config must be used to get its
configuration details.

Approved by: portmgr (blanket)