[UPD] Enforce C++ version and improve 'setup.py' flexibility

Context

1. C++-14

Fixes aidge_core#225 (closed) for aidge_backend_cpu. The same update must be done on every module.

2. Python module compilation

Also increase 'setup.py' compilation option flexibility using environment variables. Made to work with aidge!97 (merged)

changed milestone to %aidge v0.5.0

added Compilation ⚙ label

assigned to @pineapple

enabled an automatic merge when all merge checks for c5427a25 pass

mentioned in commit d19d9f59

merged