build(exmaple): set -std to c++14 since required for switch in constexpr

[e-kwsm]

$ cd example
$ clang++ -std=c++11 -Wall -I .. -I../extra -o 5-map.exe 5-map.cc
5-map.cc:37:5: warning: use of this statement in a constexpr function is a C++14 extension [-Wc++14-extensions]
   37 |     switch(channel) {
      |     ^
5-map.cc:43:5: warning: multiple return statements in constexpr function is a C++14 extension [-Wc++14-extensions]
   43 |     return "needed for gcc 5";
      |     ^
5-map.cc:38:30: note: previous return statement is here
   38 |         case Channel::Red:   return "the red channel";
      |                              ^
5-map.cc:39:30: note: previous return statement is here
   39 |         case Channel::Green: return "the green channel";
      |                              ^
5-map.cc:40:30: note: previous return statement is here
   40 |         case Channel::Blue:  return "the blue channel";
      |                              ^
2 warnings generated.