lxc: fix build with --disable-werror
lxc: fix build with --disable-werror

Fix #2592 by defining -Wvla -std=gnu11 even if --disable-werror is set
As -std=gnu11 is always set, bump requirement on gcc from 4.6 to 4.7
(see https://gcc.gnu.org/projects/cxx-status.html#cxx11)

Signed-off-by: Fabrice Fontaine <7d88633730b8b85c1516e612893df72eae9cd341@gmail.com>

hallyn/lxc,caglar10ur/lxc,ss1h2a3tw/lxc,caglar10ur/lxc,hallyn/lxc,hallyn/lxc,ss1h2a3tw/lxc,caglar10ur/lxc,ss1h2a3tw/lxc