Fix debian buster linker problem

[5balls]

It seems debian busters clang requires this change to the CMakeLists.txt file to link correctly.

I have set clang via update-alternatives and changed this file, then it worked.

I also installed clang-11 backport to buster but if I interpret cmake output correctly it used the normal system version of clang 7.0.1..

All of this may be obsolete with the next debian stable version appearing in about a week I think, so it may be reasonable to ignore this pull request if the new debian release fixes this and this pull request may cause other problems down the line (though I don't see why it should).

[5balls]

Apparently while fixing my problem this breaks other stuff - should probably not be merged.

[MichielDerhaeg]

Sadly, how to link against the right library for std::filesystem support depends on the OS, which version of the OS, which compiler is used, etc.. Not sure if we can get it right for every case.

This is also just a dependency of the unit test. For using the library it shouldn't be needed.