add MacVim

This adds [`MacVim`](https://github.com/b4winckler/macvim) for Darwin, with all the bells and whistles enabled.

I've manually tested that this works from both the terminal and the Cocoa gui, along with all of the interpreter interfaces.

The included patch removes MacVim's reliance on the system frameworks. I had to, regrettably, use some full paths to `xcodebuild` and `xcode-select`.

Once `cscope` builds on Darwin, I intend to add support for that too.

@jwiegley I see that you use Darwin quite a bit; would you mind taking a look at this and telling me what you think?