Allow attributes in macro invocations

[ids1024]

Allows #[cfg] to be used to make certain definitions conditional. Such as for library version feature flags.

Needed for Smithay/wayland-rs#735 so it can have a feature flag for the libwayland version.

Seems to work, but still needs to be added to dlib-test, with CI to test with and without the feature.

[ids1024]

Added some testing improvements. I see some issues trying to test statics: and varargs: (#23, #24), but I think the changes here are good, and aren't causing issues that don't already exist.

I don't think those block this from being merged and released. Though a semver breaking version of the crate will probably be needed at some point to fix those.

[elinorbgr]

Is this specifically needed for this change?

[ids1024]

It shouldn't be necessary, no. It just seems a bit unusual at this point for a crate to be using the default edition.

I don't think there's any need to support Rust versions too old for the 2021 edition at this point.

[elinorbgr]

In that case, could you also update the CI accordingly? It's currently failing because of that change.

[ids1024]

I've updated CI to the first version of Rust that supports the 2021 edition.

[elinorbgr]

Just to confirm, have you tested that this changes actually makes it possible to add a feature flag to Smithay/wayland-rs#735 ?

[ids1024]

Yes. Simply adding #[cfg(feature = "libwayland_1_22")] appears to work as expected in wayland-sys if it uses this version of dlib.