Draft: Use const keyword instead of constexpr one when compiler doesn't support C++11 standard

The commit (eebb6571 "Added attribute constants for cpp") introduces constexpr C++11 keyword into the generated code.

This is the only C++11 keyword present in what otherwise would be a C++98 compliant code so I propose this code that should let Pogo generates C++98 code again without sacrificing in any way C++11 users.

So we define a macro CONSTEXPR which is expanded in constexpr when a C++11 compiler is available or in const when a C++98 compiler is available.

Unfortunately, I have tested the C++ approach but not with code generated by a Pogo with this patch applied.