I'm surprised that DFHack even builds with Clang. I'm using Homebrew's gcc, although I made sure to enable objective-c support. Apparently I failed to notice the makefile, so I'll try that (and see if I can make cmake work with it somehow).
I think the main concern with plugins is making them compatible with as many platforms as possible, but since this depends on an OS X feature, it's not as important here as with other plugins. Granted, it would be nice to support this on other platforms with the required hardware, but maintaining compatibility with all of them would be a nightmare.
Edit: Using the makefile works, although it seems to require gcc 4.2 (4.5 and up don't seem to recognize -std=gnu++11, -stdlib=libstdc++, or -Wno-ignored-attributes). I'll look into compatibility with gcc 4.5/cmake more when I get a chance.