[C++] Replace %name-prefix with %define api.prefix

`%name-prefix` has been deprecated in recent bison version (3.0?). The
correct way to set the prefix to the generated names seems to now be
`%define api.prefix`.

Fix #181

source/src/BNFC/Backend/CPP/STL/CFtoBisonSTL.hs

@@ -71,7 +71,7 @@ cf2Bison ln inPackage name cf env
  = unlines
     [header inPackage name cf,
      render $ union inPackage (positionCats cf ++ allCats cf),
-     maybe "" (\ns -> "%name-prefix=\"" ++ ns ++ "yy\"") inPackage,
+     maybe "" (\ns -> "%define api.prefix {" ++ ns ++ "yy}") inPackage,
      "%token _ERROR_",
      tokens user env,
      declarations cf,