gcc.dg/compat/struct-layout-1.exp does not supported installed-compiler testing

[Eric Botcazou]

> It's presumably a matter of adding the right libiberty configure option
> from the top-level by default.  Would you care to take a try?  It sounds
> like Ian would probably approve it, or maybe one of our build
> maintainers would.

I'll try, but I don't guarantee anything.  IIRC I tried before filing the PR 
but got lost in the configure machinery.

-- 
Eric Botcazou