Since 37b0896 stb_image isn't used anymore and recently the nix CI also
broke because of the hacky way the include directory is determined. This
is now fixed. One side effect of this is, that gBar no longer needs to
be cloned recursively.
This lifts the requirement of needing to disable the dependencies by
hand if you don't have them installed.
It also enables us to have a single package, that works for all.