Fix for Visual Studio: no constexpr

Hi!

Sadly, I need the library to compile on Visual Studio 2013, where `constexpr` is not yet available. Do you think @Nocte- that this solution is ok? An alternative would be to import `boost/config.hpp` and use their `BOOST_CONSTEXPR_OR_CONST` macro, but it seems overkill for this case since almost every compiler will optimize the variable away anyway, which is the purpose, I guess, of the `constexpr` here.

Thanks!