build: Always build gpgtar.
We use gpgtar to unpack test data, hence we always build it. If the
user opts out, we simply don't install it.
- configure.ac: Add comment.
- tests/migrations/Makefile.am (required_pgms): Make sure gpgtar is
built.
- tools/Makefile.am: Always build gpgtar, but do not install it if the
user used '--disable-gpgtar'.
- Signed-off-by: Justus Winter <justus@g10code.com>