A function whose prototype says it will return a value but doesn't
is undefined behaviour in C++.  GCC 10 will generate code that crashes
in this case.

In warnings==None mode, insterad of turning off all warnings with
`-w`, explicitly list all G++ possible warnings except for the
`no-return` warning which catches this programming error.

G++ and GCC have different warning options, so use different lists.

… into safetydance

The readme now includes the exact commands required to regenerate the
none-XXX files, no manual editing needed.