GMP test

[Paolo Bonzini]

It seems that the GMP test is always run, even when installing binutils 
or gdb.

You probably need something like
if test -d ${srcdir}/gcc && test x$have_gmp != xyes; then
   ...
fi

but I think that the whole test now belongs in the GCC subdirectory, not 
in the toplevel (it was anyway a hack for the sake of disabling Fortran).

Sorry for not speaking up earlier,

Paolo