Avoid using static global variables in Constants

[jlbabilino]

This can cause issues across shared object boundaries, only ever use constexpr in Constants.hpp.

One way to fix this is to convert this:

const MyType value;

to

const MyType& GetVal() {
  static MyType val;
  return val;
}

[calcmogul]

constexpr MyType value; will work as well as long as there's a constexpr constructor.