Use define to determine if c++11 or not

[ericcurtin]

So the developer doesn't have to worry about what version of c++ a file
is being compiled as.

[ericcurtin]

Another option is to split into three files prettyprint.hpp, prettyprint98.hpp and prettyprint11.hpp

With prettyprint.hpp being like so:

#if __cplusplus >= 201103L // we are using a compiler with c++11
#include "prettyprint11.hpp"
#else // we are using pre-c++11 compiler
#include "prettyprint98.hpp"
#endif