liblxc: Handle missing PR_CAPBSET_DROP definition

liblxc: Handle missing PR_CAPBSET_DROP definition

On distros with older headers liblxc fails to build because PR_CAPBSET_DROP is
not defined by including /usr/include/sys/prctl.h. This adds an autoconf
test and, if not present, defines it. When prctl() is called on systems that
do not support PR_CAPBSET_DROP we should expect EINVAL. This case is already
handled by the liblxc code so no further changes are needed.

Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>