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

Context

1. C++-14

Fixes aidge_core#225 (closed) for aidge_backend_opencv. 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)

CMakeLists.txt

 cmake_minimum_required(VERSION 3.18)
-set(CXX_STANDARD 14)
+
+set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS  OFF)
 
 file(STRINGS "${CMAKE_SOURCE_DIR}/version.txt" version)