the Right place to change a target default for a common compiler flag?

[Joern Rennecke]

Quoting IainS <developer@sandoe-acoustics.co.uk>:

> Hi,
>
> In the case that a compiler flag in common.opt would best be served
> with different default values on different targets.
>
> I.E. a target-dependent Init()
>
> Where can this be effected in the machinery ?
>
> I can see how to make an override - but not a default.

Set the default to a special value that indicates that the variable
has not been set by a user option.
Then make the override set the variable to the target-specific
default if the variable still has this special value.