configure.ac: Build on kernels without skb->vlan_proto correctly

configure.ac: Build on kernels without skb->vlan_proto correctly

Support for EtherType other than ETH_P_8021Q for VLAN header introduced
with commit 86a9bad3ab6b ("net: vlan: add protocol argument to packet
tagging functions") in upstream kernel since v3.10.

To support build on older kernels check for ->vlan_proto presence in
@struct sk_buff and return htons(ETH_P_8021Q) when it is missing.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>