让X下的Emacs自动剪切到clipboard

[Haojun Bao]

用下面这个变量应该就可以直接往 firefox 里 C-v 从 Emacs 里拷贝出来的东西了，
否则的话得用鼠标中键，让手离开键盘去摸鼠标很evil吧，而且还得小心不能让鼠
标选上别的文本，也很evil吧。

x-select-enable-clipboard is a variable defined in `x-win.el'.
Its value is nil

Documentation:
Non-nil means cutting and pasting uses the clipboard.
This is in addition to, but in preference to, the primary selection.

On MS-Windows, this is non-nil by default, since Windows does not
support other types of selections. (The primary selection that is
set by Emacs is not accessible to other programs on Windows.)

You can customize this variable.

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