top level: make more dependencies explicit

[Andrew Cagney]

>> The mechanism is very old (it pre-dates me as GDB release engineer). 
>> Changing it is going to involve updates to many things - snapshot 
>> scripts, release process doco, .... so won't happen overnight.
> 
> 
> Mmmm.  I'm going to start rewriting it now. >:-=

What does the GNU coding standard have to say about the release process?

I'd also be wary of a ``rewrite'', the top-level stuff iteracts with 
sub-directories in strange ways.  I think reserving the existing 
behavior (but perhaphs outside of the Makefile.in) would be a better 
incremental step.

Also, how does this compare to GCC's release process.

Andrew