Upon first glance, I can't actually see that any parts of this (or any headers included) actually need MAC defined. However,
earlier commits started producing an opt_global for independent modules builds and rS357274 started defining MAC in it since it's almost universally enabled with exception to mips, which doesn't really have a solid kernel config to represent the majority. Ideally, all of these opt_global bits should be removed from modules/ in favor of defining proper defaults in sys/conf/config.mk.
[reviewer's note]
It does occur to me that Juniper may have other need for MAC to be defined beyond what I can identify by inspecting our in-tree headers and source -- in that case, this would ideally be reflected as an unconditional default down in config.mk in their tree. This file sees little churn and shouldn't be ripe for merge conflicts, other than the recent churn as I've identified defaults that modules currently care about and moved them into common bits.