Clean up compiler warnings

[Jim Kingdon]

> As for the actual changes, they all look pretty good - especially the
> bit where you delete ``extern''s from .c files :-)

I'll take that as approval :-).  Checked in.

> So you know and don't start doing unnecessary work, the suggested list
> of warning flags for 5.0 is no more than:

I didn't mess with any warning flags.  I just used the defaults that
come with "./configure; make".  If you are saying that this will not
get cleaned up until after 5.0 I guess I'm not going to argue.

As for switches on enums, in this particular case using "default"
makes sense to me, but feel free to change it to the other way if you
want.  I'm having trouble getting very excited about it one way or the
other.