* Error even w/warnings disabled for no-return fcns
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.
* Use different lists for GCC vs G++
G++ and GCC have different warning options, so use different lists.
* Make separate file for each level, add readme
The readme now includes the exact commands required to regenerate the
none-XXX files, no manual editing needed.
* Address review comments, only adjusts G++/None
