fix middle-end/16417

[Eric Botcazou]

> I can bootstrap with a gnatbind version that is definitely not
> compatible with the version which is being built.

Yes, definitely.

> It's been reported, though, that if you set a --prefix which already
> contains a GNAT installation, this one has to be compatible (but this
> is the "installed gnatbind" only by accident).  This is not the common
> case during GCC development, I presume, although it does happen if you
> install to /usr/local and then configure without --prefix.

Yes, I can confirm that too.

-- 
Eric Botcazou