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 <fontaine.fabrice@gmail.com>