Dlang uses both integer version "levels" and arbitrary string
identifiers, and we support both, but don't mention it in the docs.
Also update a test case to pass one via declare_dependency. We already
test this kwarg for build_target.
The type information is clearly wrong as it disagrees with the
description w.r.t. generated headers.
We also rely on it accepting custom targets for the obvious reason that
we accept it in a build target too! In fact, we rely on this in the
testsuite too.
Eli Schwartz