Release numbering

[Florian Weimer]

* Richard Kenner:

> Because the difference between the next release and this one will be *far*
> less than between 3.4 and this one.  If people didn't feel this level of
> changes merited "4.0", the next level certainly wouldn't.

With the current release plan, I think 3.9 would be a good compromise
because some RTL passes still remain which hopefully can be removed
once the infrastructure on the tree level is available.  3.9 shows
that the release is a significant change from 3.4, and that it's still
work in progress in some areas (some will argue that a .0 release
indicates the same thing, of course 8-).