[FEATURE REQUEST] Add conditional compilation support to `clone!` macro parameters

[wojciech-graj]

It would be useful to be able to do the following to conditionally clone a variable (x) based on any cfg attribute

glib::clone!(
    #[cfg(feature = "...")]
    #[strong]
    x,
    ...
)

[sdroege]

I think that makes sense. Do you want to provide a PR for that?

[wojciech-graj]

I might provide a PR if nobody else has picked this up in a few months, but currently I unfortunately don't have the time.