Compilers notebook: add a way to obtain all gcc warnings.

6-InRealEnvironment/3-Compilers.ipynb

 
 Some are intentionally deactivated, and plenty others I probably don't know - especially if they were introduced since I established this list (by adapting one provided on StackOverflow...) some years ago...
 
+An answer to this [StackOverflow post](https://stackoverflow.com/questions/11714827/how-can-i-turn-on-literally-all-of-gccs-warnings) provides a command to extract the list of all warnings used by gcc (unfortunately without filtering for the language used):
+
+```shell
+gcc -Q --help=warning | sed -e 's/^\s*\(\-\S*\)\s*\[\w*\]/\1 /gp;d' | tr -d '\n'
+```
+
+
+
 ### Standard library
 
 GNU also provides its implementation of the standard C++ library, which is called **libstdc++**.