setup/dirs: adjust `get-lib-search-dirs` and use if `find-dll-dir`
setup/dirs: adjust `get-lib-search-dirs` and use if `find-dll-dir`

Add `find-dll-dir` to the end, not the beginning, so that it doesn't
override the user's directory. Improve checking for whether the DLL
directory is already covered by the library directories.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket