This creates `pim6_stubs.c`, which is intended to temporarily provide
stubbed-out definitions of some functions we don't have yet for IPv6.
This makes pim6d compile without the `PIM_V6_TEMP_BREAK` hack, and is
very important as an intermediate step to get a working environment for
further work.
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
