nitg: use libunwind instead of gperf by default for stacktraces

f54e52e activate gperf by default, but gperf is in fact quite slow.
It was unnoticed since time regressions are
tracked by valgrind (and gperf was unmonitored because run in a subprocess).

However, the effect was noticeable:

~~~
$ time nitg nitg.nit --stacktrace gperf
user	1m13.377s

$ time nitg nitg.nit --stacktrace libunwind
user	0m14.189s
~~~

Signed-off-by: Jean Privat <jean@pryen.org>