Can't set MTLLINKER_FLAGS

Trying to update my CIFilters. The project absolutely refuses to set MTLLINKER_FLAGS. Says it's already defined somewhere, who knows to what. Should I take XCode's word for it? Wassup?
I found a video from last year's WWDC which included scripts to do this. At least I'm in now and can do some debugging.
Can't set MTLLINKER_FLAGS
 
 
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