Specify PROJECT_SOURCE_DIR instead of CMAKE_SOURCE_DIR

[knoepfel]

This PR accommodates MPD projects, where PROJECT_SOURCE_DIR and CMAKE_SOURCE_DIR are not the same.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #475      +/-   ##
==========================================
- Coverage   85.43%   85.38%   -0.06%     
==========================================
  Files         144      144              
  Lines        3591     3591              
  Branches      615      615              
==========================================
- Hits         3068     3066       -2     
  Misses        315      315              
- Partials      208      210       +2     

Flag	Coverage Δ
unittests	85.38% <ø> (-0.06%)	⬇️

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see 1 file with indirect coverage changes

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