How to provide optional Spaces integration for a Feature?

The issue is that whichever module is made spaces-aware, that's the module which needs to own the various components of the feature in a Spaces sense.
There are three ways you can go.

First, just provide the Spaces integration in CT Plus. You don't need to declare a dependency on Spaces to add spaces[types][] to your .info file.

Second, you can implement hook_system_info_alter() and conditionally add the spaces types setting to the $info array. I am not sure whether this will actually work.

Third, you can do #2 in ct_plus_spaces, only you can change the module ownership of all the ct_plus components to ct_plus_spaces. Of course, now you have other strange things, like a feature which will show up as CT Plus Spaces in the Spaces features page. And a pair of features which do not update easily.

I ended up barking up this tree before, and decided the most clean thing to do was to stick with #1 or #2. Using a module to integrate a feature into Spaces is not a good practice. At this point, I would either maintain two nearly identical features, one with Spaces integration, or I would put a lot of module_exists('spaces') around the ct_plus code and let it carry the Spaces integration as unobtrusively as possible.