Requiring c++14 is not going to work for Travis (because we want to support a Qt version too old to know about the c++14 config flag) or for our AppImage build (because we want to run on distros to old to support the relevant gcc ABI). So instead we try to confine the C++ standard selection to one place (config/noconfig) and revert to c++11 if the compiler is too old to know about c++14. All of the actual SV code remains c++11, and we assume we build against an older version of capnproto when we want to perform a c++11 build.