use project relative paths instead of cmake root

[hexorer]

Using CMAKE_SOURCE_DIR points to root project and will be different than PROJECT_SOURCE_DIR when this project has been add_subdirectory'ed.

it raises these kind of errors: https://github.com/WolfEngine/wolf/actions/runs/5401883414/jobs/9812341472?pr=52#step:8:80

use project relative paths to avoid such errors.