Change the test for complaining about t_Co != 256

Before, this test would be short-circuited only if gvim was running.
This caused vim to still complain when run as "vim -r" (which apparently
loads plugins, presumably to set up the proper 'directory' path...).
So, I've changed the short circuiting to happen whenever t_Co is the
empty string, which happens to be true both for gvim (which
understandably doesn't do the terminal initialization stuff) and for vim
-r (which largely inexplicably skips terminal initialization).

plugin/CSApprox.vim

@@ -665,7 +665,7 @@ endfunction
 function! s:CSApproxImpl()
   " Return if not running in an 88/256 color terminal
   if &t_Co != 256 && &t_Co != 88
-    if &verbose && !has('gui_running')
+    if &verbose && &t_Co != ''
       echomsg "CSApprox skipped; terminal only has" &t_Co "colors, not 88/256"
       echomsg "Try checking :help csapprox-terminal for workarounds"
     endif