src: avoid tentative definitions of static data

src: avoid tentative definitions of static data

Having multiple declarations of the same static data (where one is
considered "tentative") is kind of an obscure C feature -- I didn't
even know the name of it until today. It's also forbidden in C++.

In the case of circular dependencies between static data <-> function,
the more typical pattern is to forward-declare the function, then the
data, then provide the function definition. Let's follow that pattern.