Fixes #101: python_wrapper_helper uses CMAKE_CXX_STANDARD

[daedric]

Checklist:

• [x ] Your branch is up-to-date with the master branch and tested with latest changes
• [ N/A] Extended the README / documentation, if necessary
• [ x] Code compiles correctly and runs
• [ x] Code is formatted using clang-3.9 and the provided format file
• [ x] Changelog is updated

Description

Fixes #101

pygccxml should use the same standard (even if its implementation is not complete according to their doc) than cmake. It allows for instance to use the Geometry library from Boost 1.75.

Where has this been tested?

• Platform(s): Ubuntu 20.04
• Library version: boost 1.75

Possible Drawbacks

N/A

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This change is 

[daedric]

Just got back from a week-end. I see that you added a flag to silence a warning. Do you need anything else from my side?