Backport Juman++v2 to C++11

[eiennohito]

Remove C++14 specific parts or at least make them conditionally go away when compiled in C++11 mode.
Most of C++14 features used now are constexpr in StringPiece.
Need also to fix CMake configuration.

[versusvoid]

And why exactly is it needed?

[eiennohito]

Not really, I think (because I found out that RHEL has wonderful toolkits).
The codebase is mostly C++11 with an exception of 1 or 2 places though.

[eiennohito]

I thought that there would be problems on RHEL-based distros regarding C++14, but there seems to be none. Let's be C++14 and forward until the further notice.