r189369 - Warn that -O4 is the same as -O3.

[Rafael Espíndola]

> I guess my email on this subject never went through, so let me say
> again: I would like to see -O{any number >= max} treated exactly the
> same as -Omax, for GCC compatibility. You have no idea how convenient
> it is to be able to write "gcc -O99" and have it just do the right
> thing, without having to worry about which version of gcc I'm using.
>
> It's highly *in*convenient to have to remember (or encode in
> Makefiles, which is worse) the fact that "-O4" is required for maximum
> optimization by Clang 3.4 but causes a fatal error in Clang 3.5.

You can just use -O3 for all of them.

Cheers,
Rafael