Build using --with-gmp and shared libraries

[François-Xavier Coudert]

> Basic testing done on i686-linux (built with --languages=c,fortran and
> a shared libgmp in /foo/bar, and regtested). Extended testing (which
> takes ages on my computer) in progress.
>
> OK for mainline? OK for 4.0?

*ping*

This patch has both a toplevel part and a part in gcc/, so I don't
know exactly who can approve it.

FX