Fix net-snmp ipv6 option handling when other options are also used.

[• ian]

Summary

r381568 added CONFIGURE_ARGS+=--with-out-mib-modules="mibII/ipv6", but the handling of other options can result in two different --with-out-mib-modules arguments being passed to configure, and only the second one has effect. That leads to build failures because it tries to build the old mibII/ipv6 which is no longer supported on freebsd.

This change adds mibII/ipv6 to the list of excluded modules that gets turned into the --with-out-mib-modules argument.

Test Plan

Try to build the port on freebsd 11 using WITHOUT_SMUX.

[mat]

If it did not build before, this is good, if it may have, don't forget to bump PORTREVISION.

[• ian]

In D5329#114071, @mat wrote:

If it did not build before, this is good, if it may have, don't forget to bump PORTREVISION.

This change only allows option combos that used to cause build failure to now build successfully. Any combo of options that used to build okay will still build okay, yielding the same binaries as before.