Fixing deferred warnings on SBCL 1.4.7+

[Robert Goldman]

OK, I have pushed a better solution to gitlab on cl.net, as merge 
request 95.

The history of this branch is a little messy, so I will squash-merge it 
later.  But I would definitely appreciate other eyes on.

I had hoped to localize the fix, but it was too hard for me -- getting 
it right involved excruciatingly complex dealing with evaluation at 
read, compile, and load times, and the difference between loading ASDF 
as a big single file and as an ASDF system itself.

I'll plan to merge this into master at the end of the week unless I hear 
some screams before then.

P.S. I created a milestone for a bug fix release, but mis-named it, and 
I don't seem to be able to edit it. 😜

Cheers,
r