Refactor init to avoid link bugs on macOS Monterey (#426)
Refactor init to avoid link bugs on macOS Monterey (#426)

Compilation currently fails for curl on macOS Monterey due to upstream rustc issue https://github.com/rust-lang/rust/issues/90342. To make this problem hurt users less, we can work around this by avoiding the specific issue that this bug causes.

To avoid the rustc issue, we cannot directly reference any symbol that is configured to be in a constructor linker section, which we were previously doing intentionally to work around a different rustc issue https://github.com/rust-lang/rust/issues/47384. We should be able to avoid both bugs by defining our constructor symbol as a public item in the root module, though not directly referencing it in other code. Since the root module is always used (`init` is called on-demand in key places in the code) it should not be removed by optimization.

Also add a quick unit test to make sure the constructor is still working for the platforms we have CI for.

Fixes #417.
alexcrichton/curl-rust,alexcrichton/curl-rust,alexcrichton/curl-rust