Warn about too old Emacs

[miikka]

How about showing a message if the Emacs version is too old for Spacemacs?

I just tried to install Spacemacs on Emacs 23.3.1, started Emacs and nothing happened. Took me a while to figure out what's going on. Doing nothing is nicer than getting mysterious error messages that backwards-incompability would surely cause, but it'd be even nicer to get a simple message that tells me what's wrong.

[tshemeng]

good idea

(when (version< emacs-version  "24.4")
  (message "emacs too old"))

[calebmeyer]

I don't mind PRing this change (assuming the code above isn't pseudocode), but I don't know where it should live.

[StreakyCobra]

@calebmeyer In init.el there is already a check for this with a when. This when could probably be changed to an if, and in the else clause, display a message to say emacs is to old.

[Immortalin]

@StreakyCobra @syl20bnr merge pull request and add fixed in develop label?

[AdrieanKhisbe]

(@Immortalin: just made a comment on the commit about formating :) )