Setup refactoring

I refactored the setup file a bit so that it is less redundant and thus easier to grok what it is going on. It now only depends on `NDK_LIBDIR` when not compiling as host-extension (in theory, see the paragraph below). I also changed it to use `make-pathname` rather than `string-append` for concatenating paths.

There's one thing I don't quite understand and didn't manage to solve so far: Installing with a regular (non-android) Chicken via just `chicken-install` or `chicken-install -host` still leads to building with the NDK because `(host-extension)` always returns `#f`. Is this expected?