Project can't be included as a sub-project of another CMake project

[mschuckmann]

Attempting to use CMakes fetchcontent() facility or simply including this project as a sub-project (or dependency) of another CMake project fails due to incorrect use of CMAKE_SOURCE_DIR in when setting up the include paths in src/CMakeLists.txt

The use of CMAKE_SOURCE_DIR should be replaced with CMAKE_PROJECT_SOURCE_DIR.
Additionally the use of CMAKE_CURRENT_*_DIR variables should probably be changed to use the CMAKE_PROJECT_*_DIR versions.

There maybe other similar changes required.