configure.ac: Better match for ipv6_skip_exthdr() frag_offp arg presence

configure.ac: Better match for ipv6_skip_exthdr() frag_offp arg presence

On older kernels (i.e. ones before commit 5c3a0fd7d0fc ("ip*.h: Remove
extern from function prototypes") in upstream since v3.13) we fail to
match number of arguments ipv6_skip_exthdr() correctly. Configure
chooses 3 args, while function has actually 4 args.

This happens because on these kernels tab (\t) is used for intendation
between function result type and function name.

Fix by matching either space for kernels with mentioned change or tab
for older kernels to select number of arguments correctly.

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