Use the standard to_string() functions for C++11 (#4279)

If compiling according to a C++ version before C++11, then define
std::to_string ourselves.

Add a to_wstring version as well

As std::to_string() for floating point types uses %.6f as floating
point format converter, instead of %G, it needs special care.

To preserve ftos() behavior (which is expected to use the %G format
converter), it no longer uses to_string().