Better header-only target testing.

**Is your feature request related to a problem? Please describe.**
I'm trying to create a header-only C++ library and running into a lot of places where the target's access level is hard-coded and not obtained from the `_access_level` method.

**Describe the solution you'd like**
To me this suggests that we do not test CMaize on header-only targets. We should add those kinds of tests.
