Make arx --version include git commit

[mgautierfr]

From @father-mande idea in #41

Is it possible to change the arx version to reflect git version (without large work) ... ex.
root@AS6602TaPhil:/opt/from_git/arx# git describe --tags
0.2.1-77-g41dfd11 the result for last git clone ... is not coherent with the last : 668f8aa arx commit identifier ...
so it's more clear to be sure we create the correct arx.