Reorganised C+11 feature detection a bit

and added CATCH_CONFIG_CPP11_NULLPTR for VS2015

include/external/clara.h

@@ -251,7 +251,7 @@ namespace Clara {
         template<typename ConfigT>
         struct IArgFunction {
             virtual ~IArgFunction() {}
-#  ifdef CATCH_CPP11_OR_GREATER
+#  ifdef CATCH_CONFIG_CPP11_GENERATED_METHODS
             IArgFunction()                      = default;
             IArgFunction( IArgFunction const& ) = default;
 #  endif