vim: Adds support for --with-gui option

[kenorb]

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• Adds support for --with-gui for Vim.
• Dropped x11 since I've read you're planning to get it rid soon anyway.
• Enforced Python 2 since some Python scripts from Vim aren't compatible with Python 3 and default python formula is using Python 3 (refs: File "dehqx.py", line 19: SyntaxError: Missing parentheses in call to 'print' vim/vim#2754)

[chdiza]

Specifying a gui would mean requiring X11.

Also, mac is not a valid argument to --enable-gui IIRC. You'd have to specify athena, I think.

[chdiza]

Or you could specify carbon, but that's been broken for a long time.

[kenorb]

Couldn't find valid arguments for --enable-gui in Vim sources, I just guessed that these are the name of the files such as gui_x11.c, gui_gtk.c or gui_mac.c, but it seems it's not. So if gtk is supported, should it be used instead? Or set it to auto?

[ilovezfs]

Thanks for the PR @kenorb, but we'll pass on this as we don't want to add more options to vim, especially not new GUI options. I suggest you use MacVim.

[chdiza]

The arguments are in ./configure --help. All of them require X11. The de fact GUI Vim for macOS is MacVim.