Inconsistent methods for determining location of configuration files

[Enkelmann]

Right now the makefile and the actual program use different methods for determining the folder where configuration files should be located. This works OK on Linux, but it fails badly on non-Linux systems (see discussion in #141).

Note that we have not decided yet, whether we want to officially support any OS other than Linux. But right now we do not depend on anything Linux-specific, so we should investigate whether supporting Windows and MacOS is feasible.

[Enkelmann]

This issue was fixed by PR #278. I know of a separate issue that is currently preventing compilation on Windows but it may already be possible to compile and run the cwe_checker on MacOS now. Although I did not get a chance to test that yet.