www/mod_auth_gssapi: Update to 1.6.0

It was added support for LibreSSL upstream.

While here, sort USES and add bison as build dependency
because there is an issue when building with byacc < 20170201,
more details in PR 223465.

Changes: https://github.com/modauthgssapi/mod_auth_gssapi/releases/tag/v1.6.0

PR: 223465
Submitted by: Christian Ullrich <chris@chrullrich.net> (maintainer)