Testing if the implementation can support weak pointers

[Name Here]

Hello everyone,
I was wondering if there is a way to test if the implementation we are
running on can create weak pointers. In the case of Flexichain (and I think
this might be a possibility for other libraries) using weak pointers isn't
completely required, but it's preferred to avoid leaking memory slowly.

Currently Flexichain has its own 15-line function that creates weak
pointers, which is very similar to that of trivial-garbage, except that it
returns the provided value if it cannot create a weak pointer (after
warning the user at load-time). We'd like to replace this with a call to
trivial-garbage:make-weak-pointer to avoid duplication, but we also want to
fall back to just returning the value as previously mentioned; I could do
(or (tg:make-weak-pointer value) value) as it will fall through to
returning nil, but that feels icky and then I need to figure out a similar
trick for weak-pointer-value.

If there isn't such a test, I have written a trivial implementation, simply
gathering all the #+ forms to choose a constant to return:
https://github.com/nodefunallowed/trivial-garbage/blob/master/trivial-garbage.lisp#L105
Again, this is probably good for a class of programs that uses weak
pointers in a way that isn't strictly necessary but very useful.

Thanks very much.
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