package/Makefile.in: pass --enable-ipv6 to autotargets configure when enabled

Some packages don't automatically enable IPv6 support if not configured
with --disable-ipv6, or use AC_TRY_RUN which doesn't work when cross
compiling (like curl), so explicitly configure with --enable-ipv6.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>