PR/17383: fix gcc in-srcdir build (take 2)

[Paolo Bonzini]

> Any comments for binutils or GDB folks?  Does anyone object to moving
> the directories in which we build tools for the host to a host subdir
> when srcdir=.?

Note that this only applies when gcc is present in the tree, with take 2 
of this patch (which I am also much more comfortable with).  Thanks for 
the review.

I am not committing it yet (and would have not committed it even if were 
not on 56k dialup...).

Paolo