Fix build compat with older libselinux for LXC

Fix build compat with older libselinux for LXC

Most versions of libselinux do not contain the function
selinux_lxc_contexts_path() that the security driver
recently started using for LXC. We must add a conditional
check for it in configure and then disable the LXC security
driver for builds where libselinux lacks this function.

* configure.ac: Check for selinux_lxc_contexts_path
* src/security/security_selinux.c: Disable LXC security
  if selinux_lxc_contexts_path() is missing