no out of source build

[weatherhead99]

Currently, there are hardcoded instances of ${PROJECT_DIR} or similar in many locations in the buildsystem. In particular, even when doing an out of source build (i.e.mkdir build; cd build; cmake ..; make -j 4 or similar), the binaries are placed in /bin at the top level project dir.
An out of source build is strictly not needed but is convenient for packaging etc.

This issue is fixed by #53

[weatherhead99]

possible implementation - minimal pull request fixing this in #70

please DISREGARD in favour of a better solution

[weatherhead99]

please disregard PR above, I simply want a way to do an out of source build (i.e. such that the output binaries are not placed in a subdirectory of the source folder, unless the build directory itself is a subdirectory of the source folder.