On openSUSE 12.2 (at least) xtables.h is not installed in the system-wide
include dir but in /usr/include/iptables-1.4.16.3/. This results in the
following build failure:
em_ipset.c:26:21: fatal error: xtables.h: No such file or directory

Other includers of xtables.h already call out to pkg-config