Use __has_declspec_attribute for shared builds

[donny-dont]

Clang compilation targets may support __declspec attributes. Because of that it has a __has_declspec_attribute check.

Currently cURL just assumes that __declspec is Windows only. This adds a compatibility macro and checks whether dllimport and dllexport are available.

[bagder]

Okay, but do we really want this for non-windows?

[donny-dont]

Any toolchain that's clang based could potentially use __declspec. We use it on the PlayStation and ship cURL

[bagder]

Right, they can use it but with this change they will use it - unconditionally. I'm not even sure what the significance is of using this, but it is certainly a change to how it worked without this change.

BTW, the added line is too long which why the CI turns red:

../../include/curl/curl.h:117:146: warning: Longer than 79 columns (LONGLINE)

[donny-dont]

When you build clang out for a platform you have to explicitly turn on support for __declspec so this would only affect a platform that wants to use it.

I figured I'd see if the change was palatable to upstream. If its not I'm fine with maintaining it outside of curl.

[bagder]

this would only affect a platform that wants to use it.

Ah, right. Thanks for explaining this for an ignorant like me! 😁 I'll merge...