Merge: Own rand seed

Inject a basic patform-independent reproducible pseudo-random generator when `srand_form` (thus NIT_SRAND) is used.
This will allows to have more reproductible tests on various architecture.

Related with #1387

Pull-Request: #1621
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>

diff --cc lib/standard/math.nit
Simple merge

diff --cc src/c_tools.nit
Simple merge