cutelyst: depends on C++11

thread-local storage is a C++11 feature available since gcc 4.8:
https://gcc.gnu.org/projects/cxx-status.html#cxx11

Fixes:
 - http://autobuild.buildroot.net/results/2963bd55cd7c33ded3e5a75fc86079acfae844c1



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>