Use relative cmake paths

[trexxet]

Replace CMAKE_SOURCE_DIR with relative path counterparts.

These changes should allow using the library as an external dependency with the FetchContent, as for that case CMAKE_SOURCE_DIR points to the top-level CMakeLists.txt.

[Bill-Gray]

Thank you! I know virtually nothing of cmake, and was hoping somebody more knowledgeable about it would step in here.

It appears not to have broken the AppVeyor builds (which use cmake; the Github Actions builds all rely on more 'traditional' Makefiles). Those builds don't test the actual ability to use the library with FetchContent. But I'd think that if this Works On Your Machine™ and with the AppVeyor builds, and no one points out problems, we could merge this and close the underlying issue.

[trexxet]

Thanks. There are a couple more things to fix (include paths etc.), so don't merge it now.