Merge: Portable stack-traces

Most detection of the availability of libunwind and the flags to use is moved at the C compilation time (in the Makefile) instead at the Nit compilation time.

Moreover, if libunwind is not available, then stacktraces are just disabled (instead of aborting).

Another advantage is that the generated C is more portable and can be compiled in a different system.
Thus, this will help cross-compilation and should even enable the bootstrap on libunwind-less system like raspberrypi once c_src is regenerated (cf #1149).

The option `--stacktrace` is also replaced with a simpler `--no-stacktrace` for people that want to disable it completely at C-compile time.

Close #1357 and an item in #864.

Pull-Request: #1419
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>

diff --cc share/man/nitc.md
Simple merge

diff --cc src/compiler/abstract_compiler.nit
Simple merge