These days the headers we use are in glibc.  If those are too old, you can
add the -I lines to get the kernel headers.

In file included from ../../include/linux/if_tun.h:19,
                 from lguest.c:33:
../../include/linux/types.h:13:2: warning: #warning "Attempt to use kernel headers from user space, see http://kernelnewbies.org/KernelHeaders"
lguest.c: In function ‘setup_tun_net’:
lguest.c:1456: warning: dereferencing pointer ‘sin’ does break strict-aliasing rules
lguest.c:1457: warning: dereferencing pointer ‘sin’ does break strict-aliasing rules
lguest.c:1450: note: initialized from here
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>