Building with the attached random configuration file,
net/built-in.o: In function `ip_tunnel_encap':
(.text+0x9b14e): undefined reference to `gue_build_header'
net/built-in.o: In function `ip_tunnel_encap':
(.text+0x9b168): undefined reference to `fou_build_header'
make: *** [vmlinux] Error 1