Preprocessor flags are passed in CPPFLAGS.
Debian uses it to pass hardening options (-D_FORTIFY_SOURCE)
to the compiler.

- `nnn` is all about navigational convenience so the user doesn't have to type
- the binary size increases due to readline linkage
- alternative workflow: ^G, cd, `nnn`
- readline required using the prompt and the history would stay after quit

* fix invalid command

* adding '-S' argument