Add unstable-no-link feature

[xiaopengli89]

Some frameworks may only be available on higher OS versions and some applications want to check at runtime, for example:

// Load MetalFX.framework
if AnyClass::get("MTLFXSpatialScalerDescriptor").is_none() {
    let path = ns_string!("/System/Library/Frameworks/MetalFX.framework");
    let Some(metal_fx_bundle) = NSBundle::bundleWithPath(path) else {
        println!("MetalFX.framework not found");
        return;
    };
    if !metal_fx_bundle.load() {
        println!("Load MetalFX.framework failed");
        return;
    }
}

[madsmtm]

Thanks, I've been wanting this feature

[madsmtm]

Ok, last thing, promise this time:

Can your use-case be solved by -weak_framework MetalFX instead? E.g. is it enough to add the following to your crate (using a nightly from this week):

#![feature(link_arg_attribute)]

#[link(name = "-weak_framework", kind = "link-arg", modifiers = "+verbatim")]
#[link(name = "MetalFX", kind = "link-arg", modifiers = "+verbatim")]
extern "C" {}

[xiaopengli89]

Ok, last thing, promise this time:

Can your use-case be solved by -weak_framework MetalFX instead? E.g. is it enough to add the following to your crate (using a nightly from this week):

#![feature(link_arg_attribute)]

#[link(name = "-weak_framework", kind = "link-arg", modifiers = "+verbatim")]
#[link(name = "MetalFX", kind = "link-arg", modifiers = "+verbatim")]
extern "C" {}

Yeah, it works. I'll close the pr.