`#include cstdint` needs `-std=c++11` option to build

* `%%bash` compile of `20.ipynb` needed `-std=c++11` option to run.
* However, the test could not catch this. Why? Need g++ compiler version spec?
* Compatibility cross check for complex literal?

Start with 20.ipynb and its %%bash compile cell, then inspect the test that runs it and how compiler options are configured. Reproduce the issue with and without -std=c++11 across the relevant g++ versions, including the complex-literal compatibility check. Done means the test catches the required standard or clearly documents the supported compiler behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.