bug#2504: 23.0.90; new-frame => "no font backend available."

>> I think MacOS Emacs isn't very resilient to customizations for other
>> platforms that should be ignored.  For example, my
>> 
>> default-frame-alist
>> 
>> is 
>> 
>> ((menu-bar-lines . 1) (font-backend . "xft") (font . "Bitstream Vera Sans Mono-10.5") (tool-bar-lines . 0))
>> 
>> But `M-x new-frame' causes emacs to report "no font backend available"
>> without opening a new window.
>> 

> This is no different than setting (font-backend . "garbage") on any
> platform. If you have platform specific settings in your .emacs which you
> want to share between platforms, then you need to make them conditional.

We could make it easier for the users by ignoring invalid font-backends,
so a single setting can work on all platforms.


        Stefan