nitg: Added option to disable use of libunwind to generate C stack traces

As requested by @xymus, added option in nitg to remove generation of stack traces, useful when targetting other platforms like Raspberry PI, Android, NaCl and such.

Edit : Also added a check to avoid producing a stack trace on an exit(0).

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>