Remove cgo requirements on macOS

[xiebruce]

I believe go is a powerfull language and believe that go itself can get the clipboard contents, using cgo is not go anymore.

[changkun]

removing Cgo on macOS is even more complicated than #25

There is a promising project https://github.com/ebitengine/purego but there is no clear practice on what should we do to remove the cgo requirement.

PR welcome.

[xiebruce]

maybe this article can help: https://blog.webp.se/golang-libvips-cgo-en/