"--enable-inet6" or "--disble-inet6 anymore and decides automatically
whether to enable IPv6 support.
Pointed out by Matthias Andree in private e-mail.
- use kerberos instead of kerberos5 as PKG_SUPPORTED_OPTIONS
to keep compliance with other kerberos aware packages in pkgsrc
- use the krb5 buildlink environment
Introduce support for gssapi which was also requested in pr pkg/26170 with
the according PKG_SUPPORTED_OPTIONS. gssapi will imply kerberos5.
into the bsd.options.mk framework. Instead of appending to
${PKG_OPTIONS_VAR}, it appends to PKG_DEFAULT_OPTIONS. This causes
the default options to be the union of PKG_DEFAULT_OPTIONS and any
old USE_* and FOO_USE_* settings.
This fixes PR pkg/26590.
