sed -i 's/\/config\.h/\/autoconf\.h/' *.c
I still got a bunch of build errors from interceptor.c, specifically "CHECKSUM_HW undeclared" and "too many arguments to function skb_checksum_help". Great. More googling. Turns out that CHECKSUM_HW got replaced by CHECKSUM_PARTIAL and CHECKSUM_COMPLETE (see kernel.org for details), and skb_checksum_help's argument list changed.
So more googling, and I found the terribly helpful page put up by Alexander Griesser on Cisco VPN Client and Linux Kernel 2.6.19, which includes a vpnclient-linux-2.6.19.diff file you can use for easy patching. w00t!