Changed cmake var used for builds

[bjauny]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Documentation has been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) Changes CMAKE_SOURCE_DIR to PROJECT_SOURCE_DIR in all CMakeLists.txt

Is there an open issue that this resolves? If so, please link it here. Fixes #448

What is the current behavior? (You can also link to an open issue here) Build is OK

What is the new behavior (if this is a feature change)? Build is still OK

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Other information:

[CLAassistant]

All committers have signed the CLA.

[bjauny]

Thanks for the approval!