How about something like the following.
if [ -x /usr/sbin/selinuxenabled -a -f /etc/selinux/config ]; then
. /etc/selinux/config
POLICYFILE=/etc/selinux/%{type}/policy/policy.18
RPMPOLICYFILE=$POLICYFILE.rpmnew
if [ "${SELINUXTYPE}" = "%{type}" -a /usr/sbin/selinuxenabled -a \
-e $RPMPOLICYFILE -a \
$RPMPOLICYFILE -nt $POLICYFILE ]; then
diff -q $RPMPOLICYFILE $POLICYFILE > /dev/null ||
make -C /etc/selinux/%{type}/src/policy load > /dev/null 2>&1
fi
fi