I think really I just don't want a check that is "we happen to
know that this particular option switch is supported only
from these versions onward", because as soon as we say
"OK, we can move forward to a clang 7 baseline" we either
need to search around for an equivalent clever trick, or
else we need to get rid of all this code anyway and have
the version number check. So I don't mind how we do this
as long as there's a straightforward bit of code that
currently has "6 "and "10" in it and where we can change it to
"7" and "11 "in future and it does what we intend.