lxc: fix build without stack protector

lxc: fix build without stack protector

Stack protector has been added in version 3.0.3 by
https://github.com/lxc/lxc/commit/2268c27754152aa538db2c9e3753d72d19bcd17a

However, some compilers could missed the needed library (-lssp or
-lssp_nonshared) at linking step so use ax_check_link_flag instead of
ax_check_compile_flag

Fixes:
 - http://autobuild.buildroot.org/results/0b90e7dca2984652842832a41abad93ac49a9b86

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>