Better header-only target testing.

[ryanmrichard]

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.