`HAVE_COCOA` should be used only in the places where we’re actually
depending on the Cocoa toolkit. It should not be used as a general way
of detecting building on a Darwin-based OS such as macOS.

Conversely, there are a few places in the code where we do want to
specifically detect the Cocoa toolkit (and others where we specifically
want to detect Carbon), so keep `HAVE_COCOA` and `HAVE_CARBON` around.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #2802