Synthesis attributes can be found on page 40 of UG901. The dont_merge attribute is obsolete now, and was used in the early days to prevent multiple drivers (registers) from being merged on high fanout nets. It should be ignored by all Xilinx tools, so there's no need to modify your code. But just for your information, the Xilinx equivalent of DONT_MERGE is KEEP. It will have pretty much the same effect.
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