ENABLE_JSC_MULTIPLE_THREADS isn't used in the code anymore except for its declaration in wtf\Platform.h. I noticed this while going through old Bug 26838.
Created attachment 73508 [details] remove flag which doesn't appear to be needed anymore
Comment on attachment 73508 [details] remove flag which doesn't appear to be needed anymore According to Xcode's Find in Project feature, there are 35 occurrences of "ENABLE(JSC_MULTIPLE_THREADS)" in JavaScriptCore.
Duh, sorry. PEBKAC!