fix(building/cpack_slides): Fix path for target include dir

[neumantm]

Description

Fix the path for the target include dir for the top level project.
For the top-level project the source dir is used.
This should usually not be suffixed with a includedir, as the headers are spread across the source subfolders.

Checklist

• I made sure that the markdown files are formatted properly.
• I made sure that all hyperlinks are working.

[uekerman]

Thanks for the activity. I must admit I do not yet understand your explanation. Could you sketch the hierarchy you have in mind?

[neumantm]

Let's take the example from the exercise:

.
├── CMakeLists.txt
├── Dockerfile
├── fem
│   ├── fem.cpp
│   └── fem.hpp
├── filesystem
│   ├── filesystem.cpp
│   └── filesystem.hpp
├── flatset
│   ├── flatset.cpp
│   └── flatset.hpp
├── inittimezone
├── LICENSE
├── main.cpp
├── README.md

CMAKE_CURRENT_SOURCE_DIR would be . and headers must be included from . and not ./<somedir>.