-mno-sse4
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I meant a configure or cmake switch for the project (and #ifdefed explicit SSE code), not a global compiler switch.
-mno-sse4 is the way to go (or a proper -march). 
@trevorm:
A new build where I tried to disable SSE4 with CFLAGS=-mno-sse4 CCFLAGS=-mno-sse4 in libpng.
https://kd6kxr.keybase.pub/RawTherapee_OSX_10.9_64_5.5-289-g56fff79e4.zip
It seems to run ok on my imac8,1 which has SSE4.1 (intel E8235) where an imac7,1 does not (intel T7700).