Are you cross compiling?
No

These libraries of yours are not built as part of the same project with the same settings?
No, these libraries are build separately (using separate project files). But not because I need them to be built separately. I just didn't know that it is possible to build needed projects/libraries together with the project that uses them.
How do I achieve that (that would solve my problem in a more elegant way)? How to tell qmake which header/source files belong to each library? Do I need to add the project files of the libraries to the project that uses them? If that assumption is correct, how do I do that properly?